STRUCTURE AND INHERITANCE OF SENSE AND ANTISENSE TRANSCRIPTS FROM A TRANSPOSON IN THE GREEN-ALGA CHLAMYDOMONAS-REINHARDTII

We have studied the transcription pattern of a 5700 base-pair transposon (TOC1) in Chlamydomonas reinhardtii. Northern blotting and nuclease S1 protection experiments define three classes of major TOC1 RNAs that accumulate to different levels in a number of strains and segregate independently in the progeny of crosses: class 1 RNAs are unstable near full-length sense transcripts whose 5′ end maps to the left 217 base-pair repeat of TOC1, class 2 and class 3 RNAs are large discrete chimaeric transcripts containing full-length sense (class 2) and anti-sense (class 3) copies of TOC1 Sequence motifs common to the 5′ non-transcribed regions of C. reinhardtii genes were found upstream from the putative initiation site of class 1 transcripts. A functional polyadenylation site was located in the far-right 237 base-pair repeat of TOC1 Class 1 TOC1 transcripts are initiated and probably terminated within the terminal repeats of TOC1 and may represent retrotransposition intermediates. Class 2 and 3 TOC1 transcripts co-segregate with specific TOC1 elements identified on Southern blots. The loci that control the production of high levels of class 1 transcripts could correspond to specific TOC1 elements i.e. only a few TOC1 elements are transcribed or to a regulatory locus. The accumulation of an 11,500 to 12,000 base sense transcript (class 2) is reduced two- to fourfold by the presence of a 9500 to 9700 base anti-sense transcript (class 3). In contrast the accumulation of the 5′ ends of class 1 transcripts are unaffected by the anti-sense TOC1 transcript. © 1991.