Telomeric DNA in Chironomus, a naturally telomerase-free system

Telomeres are specialized complexes of DNA and associated proteins that protect the ends of eukaryotic chromosomes. Although most organisms have short repeated DNA sequences at their telomeres, which are specified and maintained by the enzyme telomerase, there are a few exceptions. Dipteran insects, including the genus Chironomus (non-biting midges), lack telomerase and have atypical terminal sequences. Chironomids have four chromosomes – three metacentric and one telocentric. In C. pallidivittatus, long (340 bp) and complex telomere-specific repeats terminate the seven nontelocentric chromosome ends and a similar situation probably occurs in related species. Although this DNA evolves rapidly, it is shown here that some sequence features are conserved among three analyzed Chironomus species, including C. pallidivittatus. Another one of these species (C. tentans) was found to lack the long single-stranded overhangs characteristic of canonical telomeres (those maintained by telomerase). The eighth, telocentric chromosome end, on the other hand, contains 155 bp centromere-specific repeats. In this study, it is shown that these 155 bp units extend to the very end of the chromosome. Consequently, it appears that one and the same cell can use different terminating repeats at different chromosome ends. The telocentric end is also unique in that it was found to contain an active gene, probably derived from a transposon, close to the telocentric repeat array. This gene could be involved in maintenance of the adjoining telomere and requires further investigation. Previous studies have shown that gene conversion is one factor in the maintenance of telomeric DNA at nontelocentric chromosome ends. The possibility of RNA-based telomere elongation, which was suggested by earlier immunolocalization of reverse transcriptase to telomeres, was addressed here. A previously identified RNase-sensitive fraction, hybridizing to telomeric repeats was found to be a double-stranded extrachromosomal component containing both DNA and RNA and to be unique to telomeres. However, additional research is required in order to evaluate the possible role of this material in telomere maintenance