[Characteristics of structures of Drosophila polytene chromosomes formed by transposable DNA fragments]

An electron microscopic analysis of regions of Drosophila melanogaster polytene chromosomes into which DNA fragments of different genetic composition were inserted by the P element-mediated transformation was performed. In 4 of 5 regions studied with integrated DNA sequences of the hsp28-ry, hsp70-Adh, ry-hsp70-beta-gal genes new bands appeared. Apparently their generation is mainly caused by integration of the DNA fragments in interbands. Absence of a new band in transformed region in one of the stocks can be explained by fusion of the insertion with a band existed in the initial untransformed stock. Among the transformants studied, the minimum length of DNA fragment revealed as a new band is about 5 kb. DNA packing ratio of such the bands varies from 30 to 50. The activation of the inserted genes by heat shock allows to trace peculiarities of the new bands puffing. The puff sizes correlate with the length of the activated genes. If the DNA of the fragment consists of the sequence of one gene, its activation will lead to decondensation of the whole band. In the case when DNA fragment consists of 2 genes and the promoter of activated gene is situated inside the sequence, the band is splitted after gene activation at the beginning and then separated portion of the band is decondensed and puffed. The data obtained evidence that a band of polytene chromosome is not a unit of decondensation. DNA packing ratio in puffs is equal to 1.5-3.5.