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 transposo ns within the genome of Drosophila melanogaster These elements vary in size and sequence, the longest consisting of 1736 bp with 234-bp inve rted terminal repeats. The longest open reading frame in an intact S e lement could encode a 345-amino acid polypeptide. This polypeptide is homologous to the transposases of the mariner-Tc1 superfamily of trans posable elements. S elements are ubiquitous in D. melanogaster populat ions and also appear to be present in the genomes of two sibling speci es; however, they seem to be absent from 17 other Drosophila species t hat were examined. Within D. melanogaster strains, there are, on avera ge, 37.4 cytologically detectable S elements per diploid genome. These elements are scattered throughout the chromosomes, but several sites in both the euchromatin and P heterochromatin are consistently occupie d. The discovery of an S element-insertion mutation and a reversion of this mutation indicates that S elements are at least occasionally mob ile in the D. melanogaster genome. These elements seem to insert at an AT dinucleotide within a short palindrome and apparently duplicate th at dinucleotide upon insertion.