The rate of production of cytoplasmic, heterogenously sedimenting, poly(A)-containing messenger RNA is greatly reduced when cultured Drosophila cells are subjected to heat shock. A large number of discrete RNA species which do continue to appear in the cytoplasm when cells are treated at 37°C have been characterized. Twelve of these poly(A)-containing RNAs are products of the mitochondrial genome, while another five RNAs lacking poly(A) probably represent Drosophila histone mRNAs since those which have been tested hybridize specifically to the region of the polytene chromosomes which contain the histone coding sequences. Six electrophoretically separable fractions of RNA whose appearance in the cytoplasm is greatly stimulated by heat-treatment comprise the majority of the polyadenylated RNA labeled during a heat shock. In all likelihood these RNAs code for the new proteins synthesized by heat-shocked cells since they are associated almost entirely with polysomes and hybridize in situ to regions of the Drosophila chromosomes which puff in response to heat-treatment. The kinetics of synthesis of these mRNAs and the effect of different temperatures on their production have been investigated. A complex relationship has been found to exist between the chromosomal sites which form heat shock puffs and the temperature-induced mRNAs. Three sites, 63C, 67B and 95D, each show hybridization in situ mainly with unique fractions of heat-induced mRNA as expected if a one-to-one relationship between puffs and mRNAs exists. The most abundant mRNA species labels two sites, 87A and 87C, and these loci react to a lesser extent with many other RNA fractions. All the mRNAs show some complementarity with DNA in or very near to the chromocenter. These observations, together with the finding that at least 25 euchromatic chromosomal sites which do not form visible heat shock puffs also hybridize in situ to some degree with RNA labeled in heat-shocked cells, are discussed.
ASJC Scopus subject areas
- Molecular Biology