A FAMILY OF COMPLEX TANDEM DNA REPEATS IN THE TELOMERES OF CHIRONOMUS-PALLIDIVITTATUS

A family of 340-bp tandem telomere-associated DNA repeats is present i n 50- to 200-kb blocks in seven of the eight paired chromosome ends in Chironomus pallidivittatus. It consists of four main subfamilies, dif fering from each other by small clusters of mutations. This differenti ation may reflect different functional roles for the repeats. Here we find that one subfamily, D3, is consistently localized most peripheral ly and extends close to the ends of the chromosomes, as shown by its s ensitivity to the exonuclease Bal 31. The amounts of D3 are highly var iable between individuals. The repeat characteristic for D3 forms a se gment with pronounced dyad symmetry, which in single-strand form would give rise to a hairpin. Evidence from an interspecies comparison sugg ests that a similar structure is the result of selective forces. Anoth er subfamily, M1, is present more proximally in a subgroup of telomere s characterized by a special kind of repeat variability. Thus, a compl ex block with three kinds of subfamilies may occupy different M1 telom eres depending on the stock of animals. We conclude that subfamilies a re differentially distributed between and within telomeres and are lik ely to serve different functions.