Definition of a second XXXX pur regulon comprising the pur and xpt-pbuX operons plus pbuG, nupG (yxjA), and pbuE (ydhL).

@entity2642 @entity2353 @entity3625 @entity10043 @entity535 @entity72793 @entity377 @entity2774 @entity3881
In @entity3881 expression of genes or operons encoding enzymes and other proteins involved in @entity10043 synthesis is affected by @entity10043 bases and @entity2642 in the growth medium. The genes belonging to the PurR regulon (purR, purA, glyA, guaC, pbuO, pbuG, and the pur, yqhZ-folD, and xpt-pbuX operons) are controlled by the PurR repressor, which inhibits transcription initiation. Other genes are regulated by a less-well-described transcription termination mechanism that responds to the presence of @entity3625 and @entity2774 . The pur operon and the xpt-pbuX operon, which were studied here, are regulated by both mechanisms. We isolated two mutants resistant to @entity72793 in which the pur operon and the xpt-pbuX operon are expressed at increased levels in a PurR-independent manner. The mutations were caused by deletions that disrupted a potential transcription terminator structure located immediately upstream of the ydhL gene. The 5' part of the ydhL leader region contained a 63- @entity377 (nt) sequence very similar to the 5' ends of the leaders of the pur and xpt-pbuX operons. Transcripts of these regions may form a common tandem stem-loop secondary structure. Two additional genes with potential leader regions containing the 63-nt sequence are pbuG, encoding a @entity3625 - @entity2774 transporter, and yxjA, which was shown to encode a @entity10043 @entity2353 transporter and is renamed nupG. Transcriptional lacZ fusions and mutations in the 63-nt sequence encoding the possible secondary structures provided evidence that expression of the pur and xpt-pbuX operons and expression of the ydhL, nupG, and pbuG genes are regulated by a common mechanism. The new pur regulon is designated the XptR regulon. Except for ydhL, the operons and genes were negatively regulated by @entity3625 and @entity2774 . ydhL was positively regulated. The derived @entity535 sequence encoded by ydhL (now called pbuE) is similar to the @entity535 sequences of metabolite efflux pumps. When overexpressed, PbuE lowers the sensitivity to @entity10043 analogs. Indirect evidence indicated that PbuE decreases the size of the internal pool of @entity3625 . This explains why the @entity3625 - and @entity2774 -regulated genes are expressed at elevated levels in a mutant that overexpresses pbuE.
