Organization of centromere associated DNA in Chironomus pallidivittatus.

Centromeres have not yet been functionally defined in higher eu- karyotes, largely due to the huge amounts of repeated DNA in these regions of most species and difficulties of delimiting centromeric from pericentric regions. Also the latter have large quantities of mostly noncoding DNA, expressed cytologically as pericentric heterochromatin. The dipteran Chironomus pallidivittatus is advantageous in having narrow centromeric/ pericentric regions, in thin chromosome bands containing about 50 kb of centromere specific tandem 155 bp repeats. We have described a mobile element characterized by a combination of an RNA polymerase III transcrip- tion unit and a large palindrome surrounding about 100 bp, some of which is partially palindromic, and the rest of which is highly AT-rich. Similar features also characterize the functionally defined centromeres in yeast. I have also defined the border between the block of 155 bp repeats in the centromere 2 region and adjoining pericentric repetitive DNA, which I have described up to the border towards unique DNA. There are several classes of pericentric repeats with different patterns of extracentromeric distri- butions. Except for the 155 bp repeats most sequences are absent from other centromeric/pericentric regions, speaking against a role related to centromeric function for these. Finally, I have identified a second centromere specific tandem repeat, 380 bp long, restricted to one of the four centromeres, which may be a functional equivalent of the 155 bp repeat