2 SUBFAMILIES OF MDG1 RETROTRANSPOSON WITH DIFFERENT EVOLUTIONARY HISTORIES IN DROSOPHILA-MELANOGASTER

Two copies of nonmobile retrotransposon localized in D. melanogaster h eterochromatin (mdg1het) were sequenced at the 3'-end. The comparison of 2.5-kb mdg1het sequences with the sequence of cognate euchomatic tr ansposable copies (mdg1tr) revealed an intact mdg1 ORF2 encoding the p ol gene in mdg1het, and two-thirds of nucleotide substitutions in this ORF were synonymous. All the known mdg1 regulatory elements in the md g1het LTR also are conserved, in spite of numerous deletions and nucle otide substitutions elsewhere in this region. These data suggest that the mdg1het subfamily lost its mobility more recently than other funct ions were lost. The G-->A hypermutation known to occur in the reverse transcription cycle of retroviruses was detected in one mdg1het copy. The structure of the enchancerlike region in mdg1het suggests a reduce d transcription level and, therefore, transposition frequency, relativ e to mdg1tr. The number of nucleotide substitutions suggests that the time of mdg1het mobility loss was less than 0.3-0.5 Myr ago.