Larvae homozygous or hemizygous for the l(1)t435 mutation located within the early ecdysteroid puff 2B5, or carrying a deletion of the 2B5 band, die at the end of the third larval instar. In the salivary gland chromosomes of these larvae only intermoult puffs are detected. If these salivary glands are incubated in vitro with 20-OH ecdysone for 6 h the intermoult puff 68 C remains large, some early puffs (74EF and 75B) are induced to 30-40% of their normal size, other early (63F) and all late puffs (62E, 78D, 82F and 63E) are not induced at all. Puff 2B5 reaches its normal size but does not regress after 6 h incubation with 20-OH ecdysone, as it does in normal stocks. The data obtained in this study show the existence of a locus (or loci) in the band (puff) 2B5 which is necessary for the normal response of the salivary gland chromosomes to the hormone 20-OH ecdysone.
The formation of constitutive heterochromatin was studied during the embryonic development of Drosophila melanogaster, using the C-banding technique. During embryonic cleavage, C-banded material is not seen in mitotic chromosomes; the differentiation between euchromatin and heterochromatin only occurs at blastoderm. This event correlates with the establishment of position-effect variegation.
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.
Siberian Division of Russian Academy Novosibirsk 630090, Russia of Sciences +Institute of Bioorganic Chemistry of Sciences I. Probing RNA Structure by Chemical and Enzymatic Approaches . . . . .
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