Addition of ammonia or amino acids to a nitrogen-depleted medium affects gene expression patterns in yeast cells during alcoholic fermentation.

Yeast cells require nitrogen and are capable of selectively using good nitrogen sources in preference to poor ones by means of the regulatory mechanism known as nitrogen catabolite repression (NCR). Herein, the effect of ammonia or amino acid addition to nitrogen-depleted medium on global yeast expression patterns in yeast cells was studied using alcoholic fermentation as a system. The results indicate that there is a differential reprogramming of the gene expression depending on the nitrogen source added. Ammonia addition resulted in a higher expression of genes involved in amino acids biosynthesis while amino acid addition prepares the cells for protein biosynthesis. Therefore, a high percentage of the genes regulated by the transcription factors involved in the regulation of amino acid biosynthesis are more expressed during the first hours after ammonia addition compared with amino acid addition. The opposite occurs for those genes regulated by the transcription factor Sfp1p, related to ribosome biosynthesis. Although both additions include rich nitrogen sources, most NCR-regulated genes are more expressed after adding ammonia than amino acids. One of the differentially expressed genes, YBR174W, is required for optimal growth in synthetic medium.