DNA-Dependent cytoplasmic RNA polymerases I and II in insect oocytes

Transcriptional regulation in Drosophila during heat shock: a nuclear run-on analysis

We used a nuclear run-on assay as a novel approach to study the changes in transcriptional activity that take place in Drosophila melanogaster during heat shock. In response to a rapid temperature upshift, total transcriptional activity in cultured KC161 cells decreased proportionally to the severity of the shock. After extended stress at 37 degrees C (15 min or more), transcription was severely reduced, and at 39 degrees C most transcription was instantaneously arrested. However, strikingly different responses were observed for individual genes. Transcription of histone H1 genes was severely inhibited even under mild heat shock conditions. Transcription of the actin 5C gene decreased progressively with increasing temperature, while transcription of the core histone genes or of the heat shock cognate genes was repressed only under severe heat shock conditions. Transcriptional activation of the D. melanogaster heat shock genes was also investigated. In unshocked cells, hsp84 was moderately transcribed, while transcriptional activity at the other protein-coding heat shock genes was undetectable (less than 0.2 polymerases per gene). Engaged but paused RNA polymerase molecules were found at the hsp70 and hsp26 genes, but not at the other heat shock genes. The rates of transcription increased with increasing temperature with a peak of expression at around 35 degrees C. At 37 degrees C, induction was less efficient, and no induction was achieved after a rapid shift to 39 degrees C. Increased transcription of the heat shock genes was observed within 1-2 min of heat shock, and maximal rates were reached within 2-5 min. Despite very similar profiles of response, different heat shock genes were transcribed at strikingly different rates, which varied over a 20-fold range. The noncoding heat shock locus 93D was transcribed at a very high rate under non-heat shock conditions, and showed a transcriptional response to elevated temperatures different from that of protein-coding heat shock genes. An estimation of the absolute rates of transcription at different temperatures was obtained.

Developmental effects of a temperature-sensitive RNA polymerase II mutation in Drosophila melanogaster

Drosophila melanogaster possessing a temperature-sensitive (ts) mutation that maps to an X-linked locus ( RpII215 ) (the locus has also been called l(1)L5 and Ultrabithorax -like or Ubl ) encoding a subunit of RNA polymerase II are fertile at 22 degrees C but become sterile when shifted to 29 degrees C. Homozygous RpII215ts adult females shifted to 29 degrees C lay structurally normal eggs for 24 hr, after which increasing numbers of eggs are abnormal. Eggs left to develop at 29 degrees C die as morphologically normal late embryos or first instar larvae when produced by females maintained at 29 degrees C for less than 6 hr. However, eggs produced by females undergoing oogenesis at 29 degrees C for longer than 6 hr develop abnormally, displaying holes primarily in their ventral cuticle and possessing an abnormal pharyngeal apparatus. As exposure of females to 29 degrees C lengthens there is an increase in the severity of these defects. Some of the eggs can be rescued by either mating RpII215ts females to wild-type males or shifting the eggs to 22 degrees C. The percentage of eggs rescued decreases with increased length of oogenesis at 29 degrees C, up to 20 hr, at which point they are no longer rescuable. The terminal phenotype of eggs that fail to be rescued by the above procedure is less extreme than that of eggs for which no rescue attempt was made. Holes in the ventral cuticle are reduced or absent, but pattern formation is disrupted such that segments are often missing, incorrectly oriented or fused. Because the RpII215 locus encodes a subunit of RNA polymerase II, the developmental defects described above are most likely due to reduced or aberrant transcription during oogenesis and early embryogenesis. This postulated effect on transcription results, in part, from the maternal loading of a gene product(s) that is thermolabile in eggs.