TAD, A NEUROSPORA LINE-LIKE RETROTRANSPOSON EXHIBITS A COMPLEX PATTERN OF TRANSCRIPTION

Tad is a LINE-like element from Neurospora. Inactive Tad elements have been found in all strains of Neurospora examined; however: active cop ies of Tad were initially found in a single strain (Adiopodoume) colle cted 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 fr om Tad elements present in strains derived from Adiopodoume containing either high or low copy numbers of active Tad, In both kinds of strai ns we found a complex set of discrete polyadenylated transcripts, As e xpected, a full-length transcript was detected with 5' and 3' ends tha t coincide with the consensus 5' and 3' ends of the Tad element. Two a dditional transcripts with the same polarity as the full-length transc ript were identified. One of these plus-strand transcripts is 2.0 kb i n length with a 5' end identical to that of the full-length transcript and an internal polyadenylation site. The other (4.2 kb) has an inter nal start site with termination occurring at the 3' consensus end of t he element, The 2.0-kb transcript encodes all of ORF1. The 4.2 kb tran script could encode a 5' truncated version of ORF2. Two minus-strand t ranscripts were also consistently identified, These represent overlapp ing transcripts from the 3' tail of Tad. These transcripts share a sma ll intron and 3' polyadenylation site. Their 5' start sites are both i nternal to the consensus 3' end of Tad. Possible functions of this com plex array of transcripts are discussed.