Search

The retroelement Penelope isolated from Drosophila virilis has a very unusual structure and codes for reverse transcriptase and an endonuclease belonging to the UvrC type. As shown previously, Penelope is a key element in induction of the hybrid dysgenesis syndrome described in D. virilis, which also involves mobilization of several unrelated mobile element families. Here we report a successful introduction of Penelope into the D. melanogaster genome by P element-mediated transformation. In the new host genome, Penelope is actively transcribed producing major transcript which coincides with that detected in dysgenic hybrids of D. virilis. In situ hybridization on D. melanogaster polytene chromosomes and Southern blotting revealed multiple transpositions of Penelope in the transformed D. melanogaster strains. We determined the structure of six Penelope copies inserted into D. melanogaster chromosomes. Some transformed D. melanogaster strains showed dysgenesis effects similar to those observed in hybrids from D. virilis dysgenic crosses.

The mobile element Penelope is activated and mobilizes several other transposons in dysgenic crosses in Drosophila virilis. Its structure proved to be complex and to vary greatly in all examined species of the virilis group. Phylogenetic analysis of the reverse transcriptase (RT) domain assigned Penelope to a new branch, rather than to any known family, of LTR-lacking retroelements. Amino acid sequence analysis showed that the C-terminal domain of the Penelope polyprotein is an active endonuclease, which is related to intron-encoded endonucleases and to bacterial repair endonuclease UrvC, and may act as an integras. Retroelements coding for a putative endonuclease that differs from typical integrase have thus far not been known. The N-terminal domain of the Penelope polyprotein was shown to contain a protease with significant homology to HIV-1 protease. Phylogenetic analysis divided the Penelope copies from several virilis species into two subfamilies, one including virtually identical full-length copies, and the other comprising highly divergent defective copies. The results suggest both vertical and horizontal transfer of the element. Possibly, Penelope invasion recurred during evolution and contributed to genome rearrangement in the virilis species. Chromosome aberrations detected in D. virilis, which is now being invaded by Penelope, is direct evidence for this assumption.

A DNA sequence from the 9F region of Drosophila melanogaster polytene chromosomes was cloned. Sequencing the cloned region and its comparison with the known sequences of the D. melanogaster genome showed that the cloned DNA part contains gene sbr and adjacent sequences. The literature data on the structure and functions of genes TAP in humans and Mex67 in yeast are discussed. These genes are orthologous to the sbr gene of Drosophila and control mRNA export from the nucleus to the cytoplasm. The literature evidence is consistent with the recessive expression of mutation l(1)ts403 (sbr10) upon heat treatment that is manifested as impaired HSP synthesis at the posttranscriptional level. However, it fails to explain the semidominant effect of the mutation manifested in high frequency of meiotic sex-chromosome nondisjunction in heat-treated females. A comparison of amino-acid sequences corresponding to the products of the three orthologous genes, TAP, Mex67, and sbr, showed that the sbr gene product of Drosophila is more similar to the human TAP factor than to the Mex67 factor in yeast.

Genomic libraries were obtained from species belonging to the "virilis" group of Drosophila. Several copies of Penelope elements were isolated from these libraries by using a D. virilis Penelope clone as a probe. The elements were sequenced, and their structure was determined. The geographical distribution of this family of mobile elements in closely related species of the group was studied in detail. Cytological localization of the elements was also carried out. The high variability observed between different copies of Penelope is probably due to recombination between individual copies. The role of these elements in the evolution of closely related species is discussed.

The His(-)-->His+ spontaneous reversion of the Salmonella typhimurium alleles hisD3052 and hisG46 was studied. The expected jackpot distribution of His+ revertants was not observed in the fluctuation tests. The experimental distributions were close to Poisson. It was also shown that the mean number of His+ reversion events and the mean number of revertants per plate were similar. These data demonstrate that the His+ reversion occurred under the influence of histidine starvation. At the same time, kanamycin resistant mutants had the jackpot distribution. Selection for His+ revertants did not increase the Kans-->Kanr mutations.