We have compared the accumulation of 5S RNA and tRNA in oocytes of Pleurodeles waltl with the corresponding process previously studied in Xenopus laevis. 5S RNA synthesis is regulated similarly in both species since different families of 5S RNA genes are transcribed in oocytes and in somatic cells of P. waltl, as in those of X. laevis. Previtellogenic oocytes of P. waltl contain only one prominent kind of storage particles (thesaurisomes). In contrast, X. laevis oocytes of the same size contain two major classes of thesaurisomes, sedimenting at 42S and 7S. The more abundant particles found in P. waltl oocytes are homologous to the larger thesaurisomes (42S) of X. laevis, but they have a lower sedimentation coefficient and a higher tRNA/5S RNA molar ratio than their X. laevis counterparts. Small amounts of particles which we think to be homologous to the 7S particles of X. laevis are present in previtellogenic oocytes of P. waltl. Therefore, the storage function of the 7S particle protein (TFIIIA) is only marginal in this species. In X. laevis oocytes TFIIIA has a second function. It acts as a positive transcription factor involved in the developmentally regulated expression of the 5S RNA genes. In X. laevis expression of the oocyte-type 5S RNA genes is accompanied by a massive accumulation of TFIIIA. This is not the case in P. waltl.

Mature amphibian oocytes contain large amounts of ribosomes and tRNA. Early studies have shown that growing oocytes of the anuran Xenopus laevis accumulate the components of their protein-synthesizing machinery in an unusual fashion. In this respect, a clear distinction must be made between 28S, 18S and 5 ·8S RNA on the one hand and 5S RNA and tRNA on the other hand. The genes coding for the former types of RNA are amplified at the beginning of the oocyte growth (Brown & Dawid, 1968; Gall, 1968). The 5S RNA and tRNA genes are not amplified (Brown & Dawid, 1968). However, the oocytes increase their production capacity for 5S RNA by activating 5S RNA genes which are repressed in somatic cells (Wegnez et al. 1972; Ford & Southern, 1973). The amplified genes are not actively transcribed during the early phase of the oocyte growth, called previtellogenesis (Ford, 1971; Mairy & Denis, 1971). In contrast, the 5S RNA and tRNA genes are active from the beginning of oogenesis. As a result, previtellogenic oocytes accumulate 5S RNA and tRNA in large molar excess with respect to 28S, 18S and 5 ·8S RNA (Ford, 1971; Mairy & Denis, 1971). 5S RNA and tRNA made in excess by small oocytes are stored in two kinds of nucleoprotein particles (thesaurisomes), sedimenting at 42S and 7S (Ford, 1971; Denis & Mairy, 1972; Picard & Wegnez, 1979).

The main features of RNA accumulation in oocytes first described in X. laevis (gene amplification and derepression; presence of two kinds of thesaurisomes), have also been found in several teleost species (Vincent et al. 1969; Mazabraud et al. 1975; Denis & Wegnez, 1977; Denis et al. 1980; Mashkova et al. 1981; Denis & le Maire, 1983). Less is known about RNA accumulation in the oocytes of urodeles. This is surprising since many studies have been devoted to RNA transcription in these cells because of the large size of their chromosomes. No information is available concerning the existence of a dual 5S RNA gene system in urodeles, although the lampbrush chromosome loops which transcribe 5S RNA have been identified in Notophthalmus viridescens (Pukkila, 1975). Urodele oocytes are known to amplify their ribosomal genes (Brown & Dawid, 1968), and to contain thesaurisomes similar in composition to the 42S particles of X. laevis (Kloetzel et al. 1981). However, it has been reported that the oocytes of the newts Triturus vulgaris and T. cristatus do not contain 7S particles (Barrett et al. 1984).

The latter observation deserves interest because the protein component of the X. laevis 7S particles plays a crucial role in the developmentally regulated transcription of 5S RNA. This protein is, in fact, a positive transcription factor, called TFIIIA (Honda & Roeder, 1980; Pelham & Brown, 1980). TFIIIA binds to an internal control region of both kinds of 5S RNA genes, thereby increasing their transcription rate by RNA polymerase III (Engelke et al. 1980; Sakonju et al. 1981). The amount of TFIIIA present in the cells is thought to regulate the transcription of the 5S RNA genes (Honda & Roeder, 1980). The oocyte-type 5S RNA genes are transcribed only when TFIIIA is very abundant. This is what happens in oocytes. When TFIIIA is present in limited amounts, as in somatic cells, only the somatic-type 5S RNA genes are transcribed. The mechanism by which TFIIIA selectively activates the oocyte 5S RNA genes has been the subject of many studies. According to a simple model, TFIIIA is supposed to bind more tightly to the somatic 5S RNA genes than to the oocyte ones (Sakonju & Brown, 1982; Wormington et al. 1983; Brown & Schlissel, 1985a). Therefore, only trace amounts of TFIIIA would be sufficient to ensure transcription of the somatic 5S RNA genes. Transcription of the oocyte 5S RNA genes would require a much higher concentration of TFIIIA. However, this explanation is no longer acceptable since recent studies have shown that TFIIIA has no preferential affinity in vitro for the somatic 5S RNA genes (McConkey & Bogenhagen, 1988). Additional factors such as TFIIIC are probably involved in the selective transcription of the 5S RNA genes (McConkey & Bogenhagen, 1988; Wolffe, 1988). The crucial factor in this process is apparently the differential stability of the transcription complexes, which in turn depends on the concentration of TFIIIA and TFIIIC in the various types of cells (Wolffe, 1988; Wolfe & Brown, 1988).

The absence of 7S particles in the oocytes of Triturus (Barrett et al. 1984) is intriguing. We wished to confirm this observation in another urodele species, Pleurodeles waltl. We also wanted to know if different families of 5S RNA genes are transcribed in oocytes and in somatic cells of this animal. We found that in the latter respect P. waltl does not differ from other amphibians. We also report that previtellogenic oocytes of P. waltl contain small amounts of particles that are most probably homologous to the 7S particles of X. laevis (Picard & Wegnez, 1979) and T. tinea (Denis et al. 1980).

Previtellogenic oocytes were obtained from immature ovaries of P. waltl (11 –13cm from snout to tailtip). The largest oocytes in these animals were in late previtellogenic period (200 –300 μm in diameter). Total RNA was purified from whole ovaries and analysed as described (Mairy & Denis, 1971). Post-mitochondrial extracts of ovaries were fractionated by polyacrylamide gel electrophoresis or sucrose density centrifugation (Denis & Mairy, 1972; Mazabraud et al. 1975). Aliquots of the sucrose gradients were analysed for u.v.-absorbance, RNA content (Mazabraud et al. 1975), protein content (Laemmli, 1970), and cross-reactivity with an antiserum against X. laevis TFIIIA (Lagaye et al. 1988). The thesaurin a/thesaurin b molar ratio in the storage particles was determined by scanning the stained electrophoregrams in visible light. When applied to X. laevis 42S particles, this method gives a thesaurin a/thesaurin b absorbance ratio of 2 ·5, which corresponds to the expected stoichiometry (two moles of thesaurin a per mole of thesaurin b; Picard et al. 1980).

Liver and ovary 5S RNA purified as described (Denis & Wegnez, 1977) was end-labelled with pCp (Peattie, 1979) and submitted to electrophoresis in 8% polyacrylamide gels containing 7 M-urea. Autoradiography of the gels revealed the presence of one major band in the case of liver 5S RNA and five bands in the case of ovary 5S RNA. All visible bands were eluted from the gels and sequenced according to Peattie’s procedure (Peattie, 1979).

The number of 5S RNA and tRNA genes in somatic cells of P. waltl was determined by hybridizing erythrocyte DNA with saturating amounts of end-labelled 5S RNA or tRNA (Denis & Wegnez, 1977). The gene number was calculated from the hybridization values thus obtained and the DNA content of a P. waltl somatic cell (25 pg; Olmo, 1973).

The sequence of P. waltl somatic 5S RNA is presented in Fig. 1 according to the secondary structure model of De Wachter et al. (1982). The sequence shown is that of the only component that could be obtained from liver cells in sufficient amount to be analysed. As indicated in Fig. 1, oocyte 5S RNA is a heterogeneous population of molecules, differing by their length and/or by their sequence. A minor component, amounting to approximately 1 % of the material purified from ovaries, has the same sequence as somatic 5S RNA. Given its §mall abundance, this component might in fact originate from the accessory cells of the gonads. All other 5S RNA molecules of ovary origin differ by their sequence from somatic 5S RNA (Fig. 1). Among the four major subtypes of 5S RNA that could be purified from ovaries and sequenced, two components (Nos 3 and 4) are shorter versions of component No 2 (Fig. 1). We ascribe the length heterogeneity of oocyte 5S RNA to post-transcriptional modifications similar to those occurring in X. laevis oocytes (Denis & Wegnez, 1973). We conclude from the data presented in Fig. 1 that at least three different families of 5S RNA genes are transcribed in P. waltl oocytes. Another family is transcribed in liver cells. The first three families of genes differ from the fourth one at four positions (Nos 30, 40, 92 and 113), four positions (Nos 30, 40, 47 and 92) and five positions (Nos 30, 40, 47, 60 and 92), respectively.

Fig. 1.

Nucleotide sequence of somatic 5S RNA from P. waltl. The sequence is arranged according to the secondary structure model of De Wachter et al. (1982). Four distinct components (numbered 1 to 4) have been detected in oocytes, differing from somatic 5S RNA at the positions shown in the inset. S stands for G or C. The nonstandard base pair U:U is indicated by a lozenge instead of a dot for the G: C, A: U and G: U base pairs.

Fig. 1.

Nucleotide sequence of somatic 5S RNA from P. waltl. The sequence is arranged according to the secondary structure model of De Wachter et al. (1982). Four distinct components (numbered 1 to 4) have been detected in oocytes, differing from somatic 5S RNA at the positions shown in the inset. S stands for G or C. The nonstandard base pair U:U is indicated by a lozenge instead of a dot for the G: C, A: U and G: U base pairs.

Previtellogenic oocytes of P. waltl contain relatively little 5S RNA. The tRNA/5S RNA mass ratio ranges from 2:1 to 4:1 in these oocytes, instead of 1:1 to 1 ·-5:1 in those of X. laevis (Mairy & Denis, 1971). Fractionation of P. waltl ovary homogenates by sucrose density centrifugation reveals the presence of one major class of 5S RNA- and tRNA-containing particles (Fig. 2A), sedimenting like dimers of X. laevis thesaurisomes (Picard et al. 1980). The 25S particles of P. waltl have the same overall composition as the 42S particles of X. laevis (Picard et al. 1980), since they contain four major RNA and protein molecules (5S RNA, tRNA, thesaurin a and thesaurin b; Fig. 2). We find two molecules of thesaurin a per molecule of thesaurin b in the particles of P. waltl as in those of X.laevis, T. tinea and T. cristatus (Picard et al. 1980; Denis et al. 1980; Kloetzel et al. 1981). However, the P. waltl particles differ from the X. laevis ones in several respects. The former particles have a higher tRNA/5S RNA molar ratio (5:1 to 6:1 instead of 3:1; Fig. 2A), and a higher protein/RNA mass ratio (2:1 instead of 1 ·4:1). Thesaurins a and b of P. waltl have a slightly higher molecular mass than the corresponding proteins of X. laevis (53 ×103Mr instead of 50 ×103 for thesaurin a; 41 ×103Mr instead of 40 ×103Mr for thesaurin b).

Fig. 2.

RNA and protein content of a post-mitochondrial extract of P. waltl immature ovaries. The ovaries from five juvenile females were homogenized in 750 μl of 50 mm-Tris-HCl, pH7 ·5, 25 mw KC1, 5mm MgCl2. The supernatant of a low-speed centrifugation (10 000 rpm for 10 min) was layered on top of a 15 –30% sucrose density gradient made up in the homogenization buffer and spun at 38000rpm for 5 hr in a SW41 rotor. Aliquots of the gradient fractions were either extracted with phenol and analysed for RNA content in 7 ·2% polyacrylamide gels (A) or analysed for protein content in 10 ·4 % polyacrylamide gels containing 0 ·1 % sodium dodecyl sulphate (B). The position of thesaurins a and b in fractions 13 –16 of the gradient is indicated. The position of TFIIIA in fraction 18 is marked by an arrow. This protein can be revealed by an antiserum against X. laevis TFIIIA (not shown). Volume of the fractions: 0 ·6ml.

Fig. 2.

RNA and protein content of a post-mitochondrial extract of P. waltl immature ovaries. The ovaries from five juvenile females were homogenized in 750 μl of 50 mm-Tris-HCl, pH7 ·5, 25 mw KC1, 5mm MgCl2. The supernatant of a low-speed centrifugation (10 000 rpm for 10 min) was layered on top of a 15 –30% sucrose density gradient made up in the homogenization buffer and spun at 38000rpm for 5 hr in a SW41 rotor. Aliquots of the gradient fractions were either extracted with phenol and analysed for RNA content in 7 ·2% polyacrylamide gels (A) or analysed for protein content in 10 ·4 % polyacrylamide gels containing 0 ·1 % sodium dodecyl sulphate (B). The position of thesaurins a and b in fractions 13 –16 of the gradient is indicated. The position of TFIIIA in fraction 18 is marked by an arrow. This protein can be revealed by an antiserum against X. laevis TFIIIA (not shown). Volume of the fractions: 0 ·6ml.

Approximately 5 % of 5S RNA present in an ovary extract of P. waltl sediments in the top region of the sucrose density gradients (Fig. 2A). This RNA is not free in the cell extract since no u.v.-absorbing material migrates at the same position as pure 5S RNA when the extract is submitted to polyacrylamide gel electrophoresis (Fig. 3). Instead, a small peak with nearly the same mobility as X. laevis 7S particles can be seen in the gel (Fig. 3). This peak accounts for less than 0 ·5 % of the 260 nm-absorbance detected in the ovary extract. The corresponding peak in extracts of X. laevis immature ovaries contains approximately 40 times as much u.v.-absorbing material (Fig. 3). Western blot analysis of the 4-7S fractions of the P. waltl extracts (Fig. 2A) reveals a faint band reacting with an anti-TFIIIA antiserum. This band is not visible in the stained electrophoregrams (Fig. 2B). From these data, we conclude that extracts of P. waltl previtellogenic oocytes contain small but detectable amounts of particles which are most probably homologous to the 7S particles of X. laevis, since their protein moiety cross-reacts with antibodies raised against X. laevis TFIIIA. We estimate that the latter protein is 40 times less abundant in oocytes of P. waltl than in those of X. laevis (Fig. 3). TFIIIA makes up less than 1 % of total soluble protein in extracts of P. waltl oocytes (Fig. 2B), instead of several per cent in extracts of X. laevis oocytes (Denis & le Maire, 1983).

Fig. 3.

Electrophoretic analysis of X. laevis and P. waltl ovary extracts. Two 70-μl aliquots of ovary extracts, containing the same amount of u.v.-absorbing material (0 ·54 A260nm unit) were layered on two 7 ·5 % polyacrylamide gels made up in 40 HIM Tris, 20 mm sodium acetate, pH 8 ·4. The gels were submitted to a voltage gradient of 8 volts cm-1 during 2 h and scanned at 265 nm. The arrows show the position of tRNA and 5S RNA in a gel run in parallel. The 7S peaks contain 12% and 0 ·3%, respectively, of the u.v.-absorbing material present in the X. laevis and P. waltl extracts.

Fig. 3.

Electrophoretic analysis of X. laevis and P. waltl ovary extracts. Two 70-μl aliquots of ovary extracts, containing the same amount of u.v.-absorbing material (0 ·54 A260nm unit) were layered on two 7 ·5 % polyacrylamide gels made up in 40 HIM Tris, 20 mm sodium acetate, pH 8 ·4. The gels were submitted to a voltage gradient of 8 volts cm-1 during 2 h and scanned at 265 nm. The arrows show the position of tRNA and 5S RNA in a gel run in parallel. The 7S peaks contain 12% and 0 ·3%, respectively, of the u.v.-absorbing material present in the X. laevis and P. waltl extracts.

Given the high C value of P. waltl cells (Olmo, 1973), we found it interesting to compare the number of 5S RNA and tRNA genes in this species with the corresponding gene number in other amphibians. According to our measurements, a diploid cell of P. waltl contains approximately 100000 5S RNA genes (instead of 50 000 in X: laevis and 300000 in N. viridescens;Brown & Weber, 1968; Pukkila, 1975), and 300000 tRNA genes (instead of 16000 in X. laevis;Clarkson et al. 1973). The high tRNA gene redundancy in P. waltl probably explains why the oocytes of this animal accumulate more tRNA than 5S RNA (Fig. 2).

P. waltl is the first urodele in which different 5S RNA genes are shown to be transcribed in oocytes and in somatic cells. Each anuran and teleost species studied so far differs from all others by the position of the nucleotide substitutions occurring in the two types of 5S RNA (Erdmann & Wolters, 1986; Nietfeld et al. 1988). This is also the case for P. waltl since three nucleotide substitutions that we have detected in oocyte and somatic 5S RNAs (at residue Nos 60, 92 and 113; Fig. 1) have not yet been observed in other 5S RNAs. The remaining substitutions (C →U at residue Nos 30 and 40 and G →A at residue No 47) occur at identical positions in somatic and oocyte 5S RNAs of other species (X. laevis for transition at residue No 30; T. tinca and M. fossilis for transition at residue No 40; X. laevis for transition at residue No 47). All nucleotide replacements except one (at residue No 40) are located in regions of the 5S RNA molecule which are likely to be base-paired (Fig. 1). Significantly, these substitutions tend to destabilize the secondary structure of oocyte 5S RNA (Fig. 1).

The species-specificity of the nucleotide substitutions occurring in oocyte and somatic 5S RNA genes raises questions concerning the evolution of these genes (Denis & Wegnez, 1978), and the mechanism of their expression. The somatic 5S RNA genes do not differ considerably from one species to another (Denis & Wegnez, 1978; Erdmann & Wolters, 1986). In contrast, the oocyte 5S RNA genes have evolved much more rapidly. Oocyte and somatic 5S RNAs of the six species studied so far differ in a total of 22 positions, scattered on the whole length of the molecule (Erdmann & Wolters, 1986; Nietfeld et al. 1988; this paper). Among these substitutions 14 are species-specific. Clearly, the somatic 5S RNA genes have undergone a higher selective pressure than the oocyte ones. We ascribe this difference to be the constraints imposed by the interactions between the genes and the transcription factor(s) involved in their activation. The key factor is probably the need to integrate the somatic 5S RNA genes into stable transcription complexes (Wolffe & Brown, 1988). This requires a tight structural fit between the somatic 5S RNA genes and the transcription factors. The constraint is less stringent for the oocyte 5S RNA genes. These are engaged in less stable transcription complexes (Wolffe & Brown, 1988). This implies that the oocyte 5S RNA genes have a lower affinity for some element(s) of the transcription machinery. Such a reduction in affinity can be obtained by a variety of mutations in the 5S RNA genes, leading to the observed interspecific and intraspecific heterogeneity in the sequence of oocyte 5S RNA.

P. waltl previtellogenic oocytes contain only one prominent kind of storage particles (Fig. 2). The stoichiometry of these particles is not the same as in X. laevis. The main difference lies in the tRNA/5S RNA molar ratio which significantly exceeds 3:1 in P. waltl (Fig. 2). In several teleost species, the larger thesauri-somes have also been reported to contain more than three molecules of tRNA per molecule of 5S RNA (Mazabraud et al. 1975). It follows that the RNA-protein interaction in the vertebrate thesaurisomes do not conform to a single model.

P. waltl previtellogenic oocytes contain relatively few 7S particles (Fig. 2). Therefore, the storage function of the 7S particles is only marginal in P. waltl oocytes. In fact, the 7S particle concentration in these oocytes is intermediate between that found in X. laevis and T. tinca oocytes (Picard et al. 1980; Denis et al. 1980) and that found in somatic cells (Lagaye et al. 1988). These cells contain only trace amounts of 5S RNA in association with a TFIIIA-related protein (Lagaye et al. 1988). A much larger amount of 5S RNA is associated with a ribosomal protein (L5; Steitz et al. 1988). This complex appears to be a precursor to ribosome assembly (Steitz et al. 1988).

As stated in the introduction, the 7S particle protein is also involved in regulation of 5S RNA transcription. What is thought to be crucial in this respect is the number of TFIIIA molecules per 5S RNA gene copy (Brown & Schlissel, 1985b; McConkey & Bogenhagen, 1988). In X. laevis oocytes this number is very large (5 ×105 –107; Shastry et al. 1984). Since P. waltl oocytes contain twice as many 5S RNA genes, but 40 times as few 7S particles as X. laevis oocytes, we estimate that the TFIIIA/5S RNA gene ratio is 80 times lower in P. waltl oocytes than in X. laevis ones. Therefore, P. waltl oocytes of any size probably contain several thousand TFIIIA molecules per 5S RNA gene copy. This should be enough to activate the oocyte 5S RNA genes, since in living cells of X. laevis a 10-fold molar excess of TFIIIA is apparently sufficient for this activation to occur (Brown & Schlissel, 1985b).

We thank Prof. De Wachter for his constant interest in this work, Dr M. le Maire and A. Viel for many helpful discussions and A. Gomez de Garcia for skillful technical assistance.

Barrett
,
P.
,
Johnson
,
R. M.
&
Sommerville
,
J.
(
1984
).
Immunological identity of proteins that bind stored 5S RNA in Xenopus oocytes
.
Expl Cell Res
.
153
,
299
307
.
Brown
,
D. D.
&
Dawid
,
I. B.
(
1968
).
Specific gene amplification in oocytes
.
Science (Washington)
160
,
272
280
.
Brown
,
D. D.
&
Schlissel
,
M. S.
(
1985a
).
A positive transcription factor controls the differential expression of two 5S RNA genes
.
Cell
102
,
759
767
.
Brown
,
D. D.
&
Schlissel
,
M. S.
(
1985b
).
The molecular basis of differential expression of two 5S RNA genes
.
Cold Spring Harbor Symp. quant. Biol
.
50
,
549
553
.
Brown
,
D. D.
&
Weber
,
C. S.
(
1968
).
Gene linkage by RNA-DNA hybridization I. Unique DNA sequences homologous to 4s RNA, 5 s RNA and ribosomal RNA
.
J. molec. Biol
.
34
,
661
680
.
Clarkson
,
S. G.
,
Birnstiel
,
M. L.
&
Serra
,
V.
(
1973
).
Reiterated transfer RNA genes of Xenopus laevis
.
J. molec. Biol
.
79
,
391
410
.
Denis
,
H.
&
Le Maire
,
M.
(
1983
).
Thesaurisomes, a novel kind of nucleoprotein particles
.
Subcell. Biochem
.
9
,
263
297
.
Denis
,
H.
&
Mairy
,
M.
(
1972
).
Recherches biochimiques sur 1’oogen èese. Distribution intracellulaire du RNA dans les petits oocytes de Xenopus laevis
.
Eur. J. Biochem
.
25
,
524
534
.
Denis
,
H.
,
Picard
,
B.
,
Le Maire
,
M.
&
Clérot
,
J. C.
(
1980
).
Biochemical research on oogenesis. The storage particles of the teleost fish Tinea tinea
.
Devi Biol
.
17
,
218
223
.
Denis
,
H.
&
Wegnez
,
M.
(
1973
).
Recherches biochimiques sur 1’oogen èse. Synth èse et maturation du RNA 5S dans les petits oocytes de Xenopus laevis
.
Biochtmie
55
,
1137
1151
.
Denis
,
H.
&
Wegnez
,
M.
(
1977
).
Biochemical research on oogenesis. Oocytes and liver cells of the teleost fish Tinea tinea contain different kinds of 5S RNA
.
Devi Biol
.
59
,
228
236
.
De Wachter
,
R.
,
Chen
,
M.-W.
&
Vandenberghe
,
A.
(
1982
).
Conservation of secondary structure in 5 S ribosomal RNA: a uniform model for eukaryotic, eubacterial, archaebacterial and organelle sequences is energetically favourable
.
Biochimie
64
,
311
329
.
Engelke
,
D. R.
,
Ng
,
S.-Y.
,
Shastry
,
B. S.
&
Roeder
,
R. G.
(
1980
).
Specific interaction of a purified transcription factor with an internal control region of 5S RNA genes
.
Cell
19
,
717
728
.
Erdmann
,
V. A.
&
Wolters
,
J.
(
1986
).
Collection of published 5S, 5-8S and 4-5S ribosomal RNA sequences
.
Nucleic Acids Res
.
14
,
rl
r59
.
Ford
,
P. J.
(
1971
).
Non-coordinated accumulation and synthesis of 5S ribonucleic acid by ovaries of Xenopus laevis
.
Nature, Lond
.
233
,
561
564
.
Ford
,
P. J.
&
Southern
,
E. M.
(
1973
).
Different sequences for 5S RNA in kidney cells and ovaries of Xenopus laevis
.
Nature, New Biology
241
,
7
12
.
Gall
,
J. G.
(
1968
).
Differential synthesis of the genes for ribosomal RNA during amphibian o ögenesis
.
Proc. natn. Acad. Sci. U.S.A
.
60
,
553
560
.
Honda
,
B. M.
&
Roeder
,
R. G.
(
1980
).
Association of a 5S gene transcription factor with 5S RNA and altered levels of the factor during cell differentiation
.
Cell
22
,
119
126
.
Kloetzel
,
P.-M.
,
Whitfield
,
W.
&
Sommerville
,
J.
(
1981
).
Analysis and reconstruction of an RNP particle which stores 5S RNA and tRNA in amphibian oocytes
.
Nucleic Acids Res
.
9
,
605
621
.
Laemmu
,
U. K.
(
1970
).
Cleavage of structural proteins during the assembly of the head of bacteriophage T4
.
Nature, Lond
.
227
,
680
685
.
Lagaye
,
S.
,
Barque
,
J.-P.
,
Le Maire
,
M.
,
Denis
,
H.
&
Larsen
,
C.-J.
(
1988
).
Characterization by human antibodies of two HeLa cell proteins which are related to Xenopus laevis transcription factor TFIIIA
.
Nucleic Acids Res
.
16
,
2473
2487
.
Mcconkey
,
G. A.
&
Bogenhagen
,
D. F.
(
1988
).
TFIIIA binds with equal affinity to somatic and major oocyte 5S RNA genes
.
Genes Develop
.
2
,
205
214
.
Mairy
,
M.
&
Denis
,
H.
(
1971
).
Recherches biochimiques sur I’oogen èse. Synth èse et accumulation du RNA pendant 1’oogen èse du crapaud sud-africain Xenopus laevis
.
Devi Biol
.
24
,
143
165
.
Mashkova
,
T. D.
,
Serenkova
,
T. I.
,
Mazo
,
A. M.
,
Advonina
,
T. A.
,
Timofeyeva
,
M. Ya.
&
Kisselev
,
L. L.
(
1981
).
The primary structure of oocyte and somatic 5S RNAs from the loach Misgumus fossilis
.
Nucleic Acids Res
.
9
,
2141
2151
.
Mazabraud
,
A.
,
Wegnez
,
M.
&
Denis
,
H.
(
1975
).
Biochemical research on oogenesis. RNA accumulation in the oocytes of teleosts
.
Devi Biol
.
44
,
326
332
.
Nietfeld
,
W.
,
Digweed
,
M.
,
Mentzel
,
H.
,
Meyerhof
,
W.
,
Köster
,
M.
,
Knöchel
,
W.
,
Erdmann
,
V. A.
&
Pieler
,
T.
(
1988
).
Oocyte and somatic 5S ribosomal RNA and 5S RNA encoding genes in Xenopus tropicalis
.
Nucleic Acids Res
.
16
,
8803
8815
.
Olmo
,
E.
(
1973
).
Quantitative variations in the nuclear DNA and phylogenesis of the Amphibia
.
Caryologia
26
,
43
68
.
Peattie
,
D. A.
(
1979
).
Direct chemical method for sequencing RNA
.
Proc. natn. Acad. Sci. U.S.A
.
76
,
1760
1764
.
Pelham
,
H. R. B.
&
Brown
,
D. D.
(
1980
).
A specific factor that can bind either the 5S RNA gene or 5S RNA
.
Proc. natn. Acad. Sci. U.S.A
.
77
,
4170
4174
.
Picard
,
B.
,
Le Maire
,
M.
,
Wegnez
,
M.
&
Denis
,
H.
(
1980
).
Biochemical research on oogenesis. Composition of the 42-S storage particles of Xenopus laevis oocytes
.
Eur. J. Btochem
.
109
,
359
368
.
Picard
,
B.
&
Wegnez
,
M.
(
1979
).
Isolation of a 7S particle from Xenopus laevis oocytes: a 5S RNA-protein complex
.
Proc. natn. Acad. Sci. U.S.A
.
76
,
241
245
.
Pukkila
,
P. J.
(
1975
).
Identification of the lampbrush chromosome loops which transcribe 5S ribosomal RNA in Notophthalmus (Triturus) viridescens
.
Chromosoma (Berlin)
53
,
71
89
.
Sakonju
,
S.
&
Brown
,
D. D.
(
1982
).
Contact points between a positive transcription factor and the Xenopus 5S RNA gene
.
Cell
31
,
395
405
.
Sakonju
,
S.
,
Brown
,
D. D.
,
Engelke
,
D. R.
,
Ng
,
S.-Y.
,
Shastry
,
B. S.
&
Roeder
,
R. G.
(
1981
).
The binding of a transcription factor to deletion mutants of a 5S ribosomal RNA gene
.
Cell
23
,
665
669
.
Shastry
,
B. S.
,
Honda
,
B. M.
&
Roeder
,
R. G.
(
1984
).
Altered levels of a 5S gene-specific transcription factor (TFIIIA) during oogenesis and embryonic development of Xenopus laevis
.
J. biol. Chem
.
259
,
11373
11382
.
Steitz
,
J. A.
,
Berg
,
C.
,
Hendrick
,
J. P.
,
La Branche-Chabot
,
H.
,
Metspalu
,
A.
,
Rinke
,
J.
&
Yairo
,
T.
(
1988
).
A 5S rRNA/L5 complex is a precursor to ribosome assembly in mammalian cells
.
J. Cell Biol
.
106
,
545
556
.
Vincent
,
W. S.
,
Halvorson
,
H. O.
,
Chen
,
H. R.
&
Shin
,
D.
(
1969
).
A comparison of nbosomal gene amplification in uni- and multinucleolate oocytes
.
Expl Cell Res
.
57
,
240
250
.
Wegnez
,
M.
,
Monier
,
R.
&
Denis
,
H.
(
1972
).
Sequence heterogeneity of 5S RNA in Xenopus laevis
.
FEBS Lett
.
25
,
13
20
.
Wolffe
,
A. P.
(
1988
).
Transcription fraction TFIIIC can regulate differential Xenopus 5S RNA gene transcription in vitro
.
EMBO J
.
7
,
1071
1079
.
Wolffe
,
A. P.
&
Brown
,
D. D.
(
1988
).
Developmental regulation of two 5S ribosomal RNA genes
.
Science (Washington)
241
,
1626
1632
.
Wormington
,
W. M.
,
Schlissel
,
M.
&
Brown
,
D. D.
(
1983
).
Developmental regulation of Xenopus 5S RNA genes
.
Cold Spring Harbor Symp. quant. Biol
.
47
,
879
884
.