S Elements: A Family of Tc1-like Transposons in the Genome of Drosophila Melanogaster

The S elements form a diverse family of long-inverted-repeat transposons within the genome of Drosophila melanogaster. These elements vary in size and sequence, the longest consisting of 1736 bp with 234-bp inverted terminal repeats. The longest open reading frame in an intact S element could encode a 345-amino acid polypeptide. This polypeptide is homologous to the transposases of the mariner-Tc1 superfamily of transposable elements. S elements are ubiquitous in D. melanogaster populations and also appear to be present in the genomes of two sibling species; however, they seem to be absent from 17 other Drosophila species that were examined. Within D. melanogaster strains, there are, on average, 37.4 cytologically detectable S elements per diploid genome. These elements are scattered throughout the chromosomes, but several sites in both the euchromatin and β heterochromatin are consistently occupied. The discovery of an S-element-insertion mutation and a reversion of this mutation indicates that S elements are at least occasionally mobile in the D. melanogaster genome. These elements seem to insert at an AT dinucleotide within a short palindrome and apparently duplicate that dinucleotide upon insertion.     

Complete nucleotide sequence and genome organization of a Drosophila transposable genetic element, 297

The complete nucleotide sequence of 297, a Drosophila copia-like transposable element, was determined and compared with those of other similar Drosophila elements and mammalian retrovirus proviruses. It was found that 297 contains three long open reading frames, comparable in sizes and locations with gag, pol, and env genes in the proviruses of replication-competent retroviruses in vertebrates. The first and second open reading frames of 297 exhibit sequence homologies to gag and pol, respectively, of Moloney murine leukaemia virus. In particular, as with 17.6, another Drosophila copia-like element, the second open reading frame of 297 was shown to be very similar in its entire organization to the retroviral pol gene and to consist of three enzymatic domains. By contrast, no appreciable homology was found between the third open reading frame of 297 and the retroviral env gene. It is also suggested that 297 and 17.6 are a peculiar pair of copia-like elements recently diverged from a common progenitor.     

Transposable genetic element found in the 5'-flanking region of the fibroin H-chain gene in a genomic clone from the silkworm Bombyx mori

A transposable genetic element was found in the 5'-flanking region of the fibroin H-chain gene in one of the genomic clones from the silkworm Bombyx mori. This element, named K-1.4, is about 1 X 4 X 10(3) base-pairs long, contains an open reading frame of only 225 base-pairs and has inverted repeats of 12 base-pairs at both ends. Duplication of three base-pairs seems to have occurred when this element was integrated into the silkworm genome. About 15 copies of K-1.4 are present per haploid genome of various silkworm strains. Genomic loci of some of these elements are different among different strains or even among individual offspring of the same parents. K-1.4 is present also in the genome of Bombyx mandarina. The K-1.4-related sequences are present in some species belonging to the family Saturniidae.