Complete transcriptional map of yeast chromosome XI in different life conditions

Systematic sequencing of the genome of Saccharomyces cerevisiae has demonstrated the existence of many novel genes, whose functions need to be studied. Entire chromosome sequences also offer the possibility to examine functional properties of the genome at a higher hierarchical level than the genes themselves. We used ordered DNA fragments of chromosome XI to systematically probe yeast DNA and total RNA extracted from MAT a, MAT alpha and diploid cells grown under three different conditions. Taking into account transcript sizes and uniqueness of probes, we attributed 94 transcripts to sequence-predicted open reading frames (ORFs) or tRNA genes; another 83 being tentatively assigned. The remaining 187 ORFs on chromosome XI do not correspond to transcripts detected under our conditions. More than 80% of transcripts are constitutively expressed, others are regulated by medium composition or cell type, the most frequent regulations being determined by carbon source (glycerol/glucose) or rich versus synthetic medium. Moreover, we show that transcript levels and regulation patterns are not statistically different between ORFs of unknown function, which constitute ca. 40% of the total, and previously identified genes (ca. 30%) or their structural homologues.