DNA-RNA hybrids and transcriptional activity in Chironomus polytene chromosomes

Abstract

The distribution of DNA-RNA hybrids was studied in fixed Chironomus polytene chromosomes by means of specific antibodies directed against DNA-RNA hybrids. Attention was mainly focused on the relationship between detection of hybrids and local transcriptional activity. As a model to test such a relationship Balbiani rings, whose transcriptional activity was experimentally modified, and a new set of puffs induced by heat shock (t-puffs), were chosen. With a few exceptions DNA-RNA hybrids appeared, and when air-drying of the slides was avoided, in actively transcribing loci. When the slides were air dried, the antibody detected no DNA-RNA hybrids unless the preparations were submitted to conditions promoting denaturation and subsequent reannealing, or to a mild pronase digestion. Denaturation/renaturation led to the detection of DNA-RNA hybrids in heat-shock-induced as well as in heat-shock-inactivated loci. On the other hand, a mild pronase digestion led to the detection of hybrids only in the heat-shock-induced puffs. These results strongly suggest that in air-dried preparations the hybrids are masked by protein and that they are unmasked by a mild pronase digestion.