CENTROMERIC POLYMERASE-III TRANSCRIPTION UNITS IN CHIRONOMUS-PALLIDIVITTATUS

Cp1 is a polymorphic short interspersed repeat (SINE) which is distrib uted over the whole genome of the dipteran Chironomus pallidivittatus, and is particularly abundant in the centromeres. It contains two diff erent sequence modules, one of which, the B module, has a polymerase I II internal control region (ICR) typical for tRNA genes (A and B box). Such sequence motifs are common in SINEs and assumed to function in R NA-mediated transposition. In the present case, however, several struc tural features speak for another role. An investigation of the transcr iption of the B module shows that it encodes a 99 nt RNA species in vi vo, Cp1-RNA, terminating within the module. The transcription unit is likely to have evolved from a pre-tRNA gene and the transcript has seq uence similarities to non-processed pre-tRNA. Most of the in vitro tra nscription is eliminated by deletion or substitution mutation of an up stream TATA box, present within the B module, as well as by changing e ither the A or B box. The properties of the transcript suggest that it does not have a role in transposition but may have some other functio n, perhaps in the centromere.