Interbands of polytene chromosomes: binding sites and start points for RNA polymerase B (II)

Polytene chromosomes of different chironomids, i.e., Chironomus tentans, C. melanotus and Glyptotendipes barbipes were isolated from salivary glands in a native state. These chromosomes were treated in vitro either mechanically or with different ionic strengths to modify them structurally as to yield different degrees of decondensation of the compact bands. Treated and untreated polytene chromosomes were lightly fixed with formaldehyde and stained by indirect immunofluorescence for RNA polymerase B. The distribution of this enzyme in bands, interbands, puffs and centromeric heterochromatin was scored and compared with that of histone H2B. The results indicate that failure to observe an antigen in condensed regions of chromatin does not necessarily imply its absence. Decondensation of bands, for example, leads to massive uncovering of histone H2B antigen, which appears to be masked in the bands of untreated polytene chromosomes. No evidence, however, of a corresponding unmasking of RNA polymerase B molecules was observed, indicating that few if any enzyme molecules are trapped in bands. Thus binding sites for RNA polymerase B and start points for transcriptional activity of the enzyme appear always to be the interband regions.