3H-Uridine labelling patterns in the Drosophila melanogaster salivary gland chromosomes X, 2R and 3L

TLDR: There is a correlation between puff size and RNA synthetic activity, so that 3H-uridine incorporation rate increases directly with puff volume, and it is suggested that many regions of “small puffs” represent chromosome loci with constant transcriptional activity.

Abstract: RNA-synthesizing regions in the polytene chromosomes of D. melanogaster were located; the level of 3H-uridine incorporation in each of these regions was determined. — It was found that the number of RNA-synthesizing regions is much larger than the number of puffs: almost all the chromosome regions are labelled except dense bands. In most cases, “non-puffed” regions label 10 to even 100 times weaker than prominent puffs. Typical puffs, i.e. loci with increased chromosome diameter, together contain about 50% 3H-uridine incorporated into the chromosomes. — In general, there is a correlation between puff size and RNA synthetic activity, so that 3H-uridine incorporation rate increases directly with puff volume. However, within each class individual puffs differ in labelling intensity by many times. It is suggested that many regions of “small puffs”, including interbands, represent chromosome loci with constant transcriptional activity.