Comparative analysis of four parameters involved in puffing activity along chromosome arm 2L of D melanogaster.

Autoradiographic and immunofluorescent techniques have been used to analyse the relationship between puffing, transcription and occurrence of DNA/RNA hybrids in D melanogaster salivary gland chromosome 2L. Experiments of 3H-uridine incorporation have indicated that similar rates of RNA synthesis are observable in well developed puffs as well as in some diffuse bands and interbands. On the other hand, puffs of similar size incorporate 3H-uridine at quite different rates. The presence of RNA polymerase II seems to follow a coincident pattern with that of 3H-uridine incorporation. Our results indicate that the rate of transcription does not determine either the formation of a puff or its potential size. Instead, we have found a positive correlation between the amount of DNA/RNA hybrids and puff size, independently of the transcription rates. Transient accumulation of transcribed RNAs in their chromosomal compartment could therefore play a relevant role in the determination of puff size.