Two dominant temperature-sensitive (DTS) lethal mutants of Drosophila melanogaster are Pros261 and Prosβ21, previously known as DTS5 and DTS7. Heterozygotes for either mutant die as pupae when raised at 29°, but are normally viable and fertile at 25°. Previous studies have identified these as missense mutations in the genes encoding the β6 and β2 subunits of the 20S proteasome, respectively. In an effort to isolate additional proteasome-related mutants a screen for dominant suppressors of Pros261 was carried out, resulting in the identification of Pros25SuDTS [originally called Su(DTS)], a missense mutation in the gene encoding the 20S proteasome α2 subunit. Pros25SuDTS acts in a dominant manner to rescue both Pros261 and Prosβ2 1 from their DTS lethal phenotypes. Using an in vivo protein degradation assay it was shown that this suppression occurs by counteracting the dominant-negative effect of the DTS mutant on proteasome activity. Pros25 SuDTS is a recessive polyphasic lethal at ambient temperatures. The effects of these mutants on larval neuroblast mitosis were also examined. While Prosβ21 shows a modest increase in the number of defective mitotic figures, there were no defects seen with the other two mutants, other than slightly reduced mitotic indexes.
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