CDNAS DERIVED FROM PRIMARY AND SMALL CYTOPLASMIC ALU (SCALU) TRANSCRIPTS

We have isolated and sequenced twenty-six cDNAs derived from primary A lu transcripts. Most cDNAs (22/26) sequenced end in multiple T residue s, known to be at the termination for RNA polymerase III-directed tran scripts. We conclude that these cDNAs were derived from authentic, RNA polymerase III-directed primary Alu transcripts. Sequence alignment o f the cDNAs with Alu consensus sequences show that the cDNAs belong to different, previously described Alu subfamilies. The sequence variati on observed in the 3' non-Alu regions of each of the cDNAs led us to c onclude that they were derived from different genomic loci, thus demon strating that multiple Alu loci are transcriptionally active. The subf amily distribution of the cDNAs suggests that transcriptional activity is biased towards evolutionarily younger Alu subfamilies, with a stro ng selection for the consensus sequence in the first 42 bases and the promoter B box. Sequence data from seven cDNAs derived from small cyto plasmic Alu (scAlu) transcripts, a processed form of Alu transcripts, also have a similar bias towards younger Alu subfamilies. About half o f these cDNAs are due to processing or degradation, but the other half appear to be due to the formation of a cryptic RNA polymerase III ter mination signal in multiple loci. Using our sequence data, we have iso lated a transcriptionally active genomic Alu element belonging to the Ya5 subfamily. In vitro transcription studies of this element suggest that its flanking sequences contribute to its transcriptional activity . The role of flanking sequences and other factors involved in transcr iptional activity of Alu elements are discussed. (C) 1997 Academic Pre ss Limited.