The Xenopus oocyte has a distinguished history as an in vivo host for the deposition by microinjection of a variety of macromolecules including DNA (Colman, 1984), RNA (Soreq, 1985), proteins (Dingwall, Sharnick & Laskey, 1982) and polysaccharides (Paine, Moore & Horowitz, 1975). In these cases, the oocyte has simply either provided an efficient production system for a product encoded by an injected nucleic acid (Soreq, 1985; Krieg et al. 1984) or has been used to investigate mechanisms regulating RNA (De Robertis, Lienhard & Parison, 1982) and protein (Gurdon, 1970; Davey, Dimmock & Colman, 1985) targeting within a living cell. However, microinjection also provides a method for perturbing or simulating processes that occur normally within the oocyte, in order to understand their molecular nature and regulation. For example, Bienz & Gurdon (1982) during their investigations of heat-shock translational control in Xenopus oocytes, argued that the heat-shock genes encoding the 70 × l03Mr heat-shock protein (hsp 70) must be constitutively active in oocytes; this was in contrast to the behaviour of injected Drosophila hsp 70 genes in unstressed oocytes. Bienz (1984) was subsequently able to simulate the natural situation and confirm her prediction by cloning and then injecting the Xenopus hsp 70 gene into oocytes

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