ABSTRACTPyrimidines are important nucleic acid precursors which are constantly synthesized, degraded, and rebuilt in the cell. Four degradation pathways, two of which are found in eukaryotes, have been described. One of them, theURCpathway, has been initially discovered in our laboratory in the yeastLachancea kluyveri. Here, we present the global changes in gene expression inL. kluyveriin response to different nitrogen sources, including uracil, uridine, dihydrouracil, and ammonia. The expression pattern of the knownURCgenes,URC1-6, helped to identify nine putative novelURCgenes with a similar expression pattern. The microarray analysis provided evidence that both theURCandPYDgenes are under nitrogen catabolite repression inL. kluyveriand are induced by uracil or dihydrouracil, respectively. We determined the function ofURC8, which was found to catalyze the reduction of malonate semialdehyde to 3-hydroxypropionate, the final degradation product of the pathway. The other eight genes studied were all putative permeases. Our analysis of double deletion strains showed that theL. kluyveriFui1p protein transported uridine, just like its homolog inSaccharomyces cerevisiae, but we demonstrated that is was not the only uridine transporter inL. kluyveri. We also showed that theL. kluyverihomologs ofDUR3andFUR4do not have the same function that they have inS. cerevisiae, where they transport urea and uracil, respectively. InL. kluyveri, both of these deletion strains grew normally on uracil and urea.
A previously undescribed pathway for pyrimidine catabolism
