CONSERVED SUBFAMILIES OF THE DROSOPHILA HET-A TELOMERE-SPECIFIC RETROTRANSPOSON

HeT-A, a major component of Drosophila telomeres, is the first retrotr ansposon proposed to have a vital cellular function. Unlike most retro transposons, more than half of its genome is noncoding. The 3' end con tains >2.5 kb of noncoding sequence. Copies of HeT-A differ by inserti ons or deletions and multiple nucleotide changes, which initially led us to conclude that HeT-A noncoding sequences are vert fluid. How ever , we can now report, on the basis of new sequences and further analyse s, that most of these differences are due to the existence of a small number of conserved sequence subfamilies, not to extensive sequence ch ange during each transposition event. The high level of sequence conse rvation within subfamilies suggests that they arise from a small numbe r of replicatively active elements. All HeT-A subfamilies show preserv ation of two intriguing features. First, segments of extremely A-rich sequence form a distinctive pattern within the 3' noncoding region. Se cond, there is a strong strand bias of nucleotide composition: The DNA strand running 5' to 3' toward the middle of the chromosome is unusua lly rich in adenine and unusually poor in guanine. Although not faced with tile constraints of coding sequences, the HeT-A 3' noncoding sequ ence appears to be under other evolutionary constraints, possibly refl ecting its roles in the telomeres.