The purR gene encodes a repressor (PurR) controlling the synthesis of the enzymes of purine biosynthesis. The subunit of PurR was identified as a 38-kDa polypeptide by SDS/polyacrylamide gel electrophoresis. Analysis of a purR-lacZ transcriptional fusion indicated that purR expression is autoregulated. This was confirmed by gel retardation and DNaseI footprinting experiments, where two PurR-binding sites were identified in the transcribed part of purR. Introduction of a purR mutation in wild-type and pur-lac fusion strains was found to abolish purine repression of all genes of the purine biosynthetic pathway except for purA.