The results of examination of the template activity of the fixed polytene chromosomes of Drosophila hydei, monitored by 3H-UTP, under in situ assay conditions, upon the use of endogenous Drosophila polymerase, exogenous Escherichia coli RNA polymerase (holoenzyme) and exogenous Drosophila RNA polymerase II (or B) have been presented. Analysis of the data reveals that the transcription patterns with the 3 enzymes are not strictly comparable with the pattern obtained under in vivo conditions. Yet, with each of the 3 conditions of assay, there is a reasonable concordance between the template activity on the single X chromosome of the male and the paired Xs of the female, as observed under in vivo. There is also, in every case, a high positive correlation between the 3H-UMP incorporation into the X chromosome and that into a specific autosome. A site-wise analysis of 3H-UMP labelling under the 3 assay conditions also reveals that for most of the regions, the sites which are highly active in vivo also show high labelling in situ, and the proportionally is maintained in both sexes. These result have been interpreted to have suggested that the hyperactivity of the male X vis-a-vis dosage compensation in Drosophila is primarily a property of the inherent organization of the X chromosome itself and is achieved through modulation in the organization, rather than exclusively through autosomal factor(s), although a secondary level of autosomal regulation has not yet been ruled out.

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