Several yeast genes produce multiple transcripts with different 3'-ends. Of these, four genes are known to produce truncated transcripts that end within the coding sequence of longer transcripts: CBP1, AEP2/ATP13, RNA 14 and SIR1. It has been shown that the level of the truncated CBP1 transcript increases during the switch to respiratory growth while that of the full-length transcript decreases. To determine whether this phenomenon is unique to CBP1, northern analysis was used to determine whether the levels of other truncated transcripts are regulated similarly by carbon source. The levels of the shortest transcripts of AEP2/ATP13 and RNA 14 increased during respiration while the shortest SIR1 transcript remained constant. However, two longer SIR1 transcripts were regulated reciprocally by carbon source. Mapping the 3'-ends of each transcript by sequencing partial cDNA clones revealed multiple 3'-ends for each transcript. Examination of the sequences surrounding the 3'-ends of the induced transcripts failed to identify a consensus sequence but did reveal weak putative 3'end formation signals in all of the transcripts. Similarly, no consensus sequence was found when the sequences surrounding the 3'-ends of the longest transcripts were compared, but again weak putative 3'-end formation signals were identified. These data are suggestive of carbon source regulation of alternative poly(A) site choice in yeast.
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