Sites of transcriptional activity in the whole set of Drosophila melanogaster polytene chromosomes have been localized by means of fluorescent antibodies against DNA:RNA hybrid molecules and compared with results on 3H-uridine incorporation obtained earlier. The majority of large and small puffs with intensive 3H-uridine incorporation demonstrate bright fluorescence. Moreover, bright fluorescence is also observed for a large number of small puffs though the intensity of 3H-uridine incorporation is low. Some prominent puffs with high levels of 3H-uridine incorporation show weak fluorescence. Condensed bands, as a rule, do not show fluorescence. The regions that look like interbands under the light microscope are not real interbands, but consist of minibands visible only in the electron microscope (EM). However, a region that has been previously studied by EM and proven to be a real interband between two thick dark bands (100B3-100B4-5) showed fluorescence. These data support previous suggestions indicating a substantial contribution of transcriptional products from small puffs and interbands to the whole transcriptional system of polytene chromosomes.

Position-effect variegation in Drosophila melanogaster is accompanied by compaction of the corresponding chromosomal regions. The compaction can be continuous, so that bands and interbands located distal to the eu-heterochromatic junction fuse into one dense block, or discontinuous, when two or more zones of compaction are separated by morphologically and functionally normal regions. In this work it was found that in both continuous and discontinuous compaction the blocks of dense material contain the immunochemically detectable protein HP1, which has previously been characterized as specific for heterochromatin. The regions undergoing compaction do not contain HP1 when they have a normal banding pattern. Thus, it may be proposed that HP1 is one of the factors involved in compaction. If two different or two identical rearrangements are combined in the same nucleus, they variegate independently. The frequency of compaction of the two rearrangements in the same nucleus corresponds to the product of the frequencies of the compact state of the individual elements. The extent of compaction (i.e. the number of bands involved in heterochromatization) of each rearrangement does not depend on the compaction pattern of the other rearranged element.

A chromosomal region subjected to position effect variegation was analysed for possible DNA under-replication. DNA clones from the vicinity of the euheterochromatin junction and from a distance of hundreds of kilobase pairs were used as probes. Formation of compact blocks of chromatin is regarded as a characteristic feature of position effect variegation. It was shown that in T (1;2) dorvar7 males undergoing position effect variegation clones representing the DNA nearest to the breakpoint in 2B7 hybridized normally in situ to the compact blocks, providing evidence against DNA underreplication. In females the same clones did not hybridize to the compact blocks. These variations in hybridization may be related to different degrees of compaction of chromosome regions in males and females. A correlation between the degree of underreplication and the level of cell polyteny was shown by Southern-blot hybridization of a DNA probe from the 2B region to DNA from an X/O strain carrying Dp (1;1)pn2b displaying position effect variegation and compaction in 94% of salivary gland cells. Almost complete underreplication of the DNA of this region was found in salivary gland cells (with a maximal degree of polyteny), intermediate underreplication was found in fat body cells (with an intermediate degree of polyteny), and replication was not disturbed in diploid cells of the larval cephalic complex.

This mini-review covers new data regarding the problem of the functional organization of polytene chromosomes: The localization of RNA synthesis in the polytene chromosome puffs, diffuse bands and interbands; The relative stability of banding pattern and its functional value; The informational content of bands.