Premature Termination of GAT1 Transcription Explains Paradoxical Negative Correlation Between Nitrogen-responsive MRNA, but Constitutive Low-level Protein Production

The first step in executing the genetic program of a cell is production of mRNA. In yeast, almost every gene is transcribed as multiple distinct isoforms, differing at their 5' and/or 3' termini. However, the implications and functional significance of the transcriptome-wide diversity of mRNA termini remains largely unexplored. In this paper, we show that the GAT1 gene, encoding a transcriptional activator of nitrogen-responsive catabolic genes, produces a variety of mRNAs differing in their 5' and 3' termini. Alternative transcription initiation leads to the constitutive, low level production of 2 full length proteins differing in their N-termini, whereas premature transcriptional termination generates a short, highly nitrogen catabolite repression- (NCR-) sensitive transcript that, as far as we can determine, is not translated under the growth conditions we used, but rather likely protects the cell from excess Gat1.