Comparison of in vivo and in vitro RNA synthesis on polytene chromosomes of Drosophila

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Abstract
A comparative radioautographic study of the RNA precursors incorporation on polytene chromosomes of Drosophila in vivo in the cells of salivary glands, and in vitro during incubation of E. coli RNA polymerase on slides with fixed chromosomes was performed. — The pattern of in vivo 3H-uridine incorporation on different sections of the chromosomes drastically differed from the in vitro 3H-UTP incorporation which seems to be much more related to DNA content of the individual small sections. In both cases puffing of the loci resulted in the increase of RNA synthesis but in vitro only 2–3 fold and in vivo much more. Hence, RNA synthesis in vitro was unspecific and did not reflect the in vivo RNA synthesis. — On the other hand, E.coli RNA polymerase completely mimics in vitro the dosage compensation phenomenon making twice as much RNA on one X-chromosome of males (1X2A) as on each of X-chromosomes of diploid (2X2A) and triploid (3X3A) females and super-females (3X2A), and the intermediate amount of RNA on each of X-chromosomes of intersexes (2X3A). It is suggested that the differences in the in vitro template activity of X-chromosomes of cells with different X∶A ratio are due to different extent of condensation of their deoxyribonucleoprotein (DNP). Yet, both male and each of female X-chromosomes bind the same amount of thymus histone FI labelled with fluorochrome which indicates that they contain the same amount of “open” regions with exposedchromosomal DNA accessible to external proteins. — On the basis of these observations a hypothesis is put forward which suggests that RNA transcription in animal chromosomes is regulated at two levels by different mechanisms; the first one controls the extent of condensation of DNP of genetic loci and determines their competence to the second mechanism which involves the action of gene-specific activator proteins. According to this hypothesis the phenomenon of dosage compensation of sex-linked genes is due to decondensation of DNP of male X-chromosome which renders its loci twice as responsive to activators as compared to the same loci in females.