Tad, a Neurospora LINE-like retrotransposon exhibits a complex pattern of transcription

Abstract

Tad is a LINE-like element from Neurospora. Inactive Tad elements have been found in all strains of Neurospora examined; however, active copies of Tad were initially found in a single strain (Adiopodoumé) collected from nature. Active Tad elements can be transferred to laboratory strains by means of crosses, heterokaryons or by transformation with cloned active elements. We have analyzed the pattern of transcripts form Tad elements present in strains derived from Adiopodoumé containing either high or low copy numbers of active Tad. In both kinds of strains we found a complex set of discrete polyadenylated transcripts. As expected, a full-length transcript was detected with 5' and 3' ends that coincide with the consensus 5' and 3' ends of the Tad element. Two additional transcripts with the same polarity as the full-length transcript were identified. One of these plus-strand transcripts is 2.0 kb in length with a 5' end identical to that of the full-length transcrit and an internal polyadenylation site. The other (4.2 kb) has an internal start site with termination occurring at the 3' consensus end of the element. The 2.0-kb transcript encodes all of ORF1. The 4.2kb transcript could encode a 5' truncated version of ORF2. Two minus-strand transcripts were also consistently identified. These represent overlapping transcripts from the 3' tail of Tad. These transcripts share a small intron and 3' polyadenylation site. Their 5' start sites are both internal to the consensus 3' end of Tad. Possible functions of this complex array of transcripts are discussed.