Nitrogen catabolite regulation of proline permease in Saccharomyces cerevisiae

The proline permease gene PUT4 has been cloned. Nitrogen-source regulation ('ammonia sensitivity') of this and at least two other amino-acid permeases is believed to occur at two distinct levels, i.e. permease synthesis and permease activity. Therefore, PUT4 transcription/messenger stability was examined in the ammonia- and proline-grown wild type as well as in mutant strains supposedly affected at only one or at both of these levels. We report transcript-level repression of proline permease synthesis in ammonia-grown cells. Repression is lifted at this level in gdhCR, gln1ts and gdhA mutants which exhibit pleiotropically derepressed permease and catabolic enzyme activities. On the other hand, the npi1 and npi2 mutations, formerly called mut2 and mut4, relieve an inactivation process which seems only to affect permeases. These mutations do not affect the detected PUT4 RNA level. The only known positive factor in proline permease regulation, the nitrogen permease reactivator protein Npr1, is believed to counteract the inactivation process on derepressing media. This protein appears to have an additional, indirect effect on PUT4 transcription/messenger stability: it would actually mediate repression via its activating effect on ammonia uptake.