Chemical and conformational changes in chromosome regions being actively transcribed.

U.V. microspectrophotometry has been used to calculate quantities of nucleic acids and proteins of complete polytene chromosomal sets and specific regions of these chromosomes. It has been found that in chromosomes the ratio of DNA to proteins is approximately 1:4. This ratio however changes when specific regions are compared. The average ratio of DNA to proteins in a puffed region (2-48B4C5) increases to 1:16 in contrast to 1:6 from the same region but in non puffed state. At the same time the RNA quantity increases by a factor of 2. thermal denaturation profiles of formaldehyde fixed chromosomes show that the Tm of this region in puffed and non puffed state differ by 10 degrees C. Moreover these profiles suggest that a large fraction of histone-bound DNA is destabilized during puffing.