Summary: The gene, encoding phosphoribosylpyrophosphate synthetase, is preceded by a leader, which is 302 bp long in and 417 bp in . A potential open reading frame (ORF) extends across the promoter and into the leader. The region between the coding region and an upstream gene () in and was cloned, sequenced and shown to encode two ORFs of unknown function. ORF 1 encodes a 23 kDa protein and ORF 2 a 31 kDa protein, as observed by denaturing PAGE of extracts of cells bearing plasmids encoding the ORFs. Both ORFs are transcribed in the same direction as the gene with ORF 2 extending into the leader. Northern blot analysis showed that the message in was on 1.3 and 2.7 kb transcripts. The shorter transcript encoded the gene only, while the longer transcript also encoded the two ORFs. Thus, the gene is transcribed from two promoters, the first promoter (P) originating upstream of ORF 1, and expressing the gene in a tricistronic operon and a second promoter (P), located within the ORF 2 coding frame, which transcribes the gene only. The transcripts encoding only were 20 times as abundant as the tricistronic transcripts under all conditions examined. This was the case whether cells containing plasmid-encoded or only chromosomally encoded copies of the region were probed for these transcripts. Derepression of the gene upon pyrimidine starvation was shown to be due to an increase in the amount of message originating from the promoter P.


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