Structural and functional organization of the gpt gene region of Escherichia coli.

The nucleotide sequence was determined for the Escherichia coli region containing the gpt gene, which encodes the enzyme xanthine-guanine phosphoribosyl transferase (XGPRT; EC 2.4.2.22). Restriction enzyme and sequence analyses have allowed us to locate precisely the gpt gene (16.9-kDal XGPRT) with respect to other genes in this region, notably phoE. Genes gpt and phoE are pointing towards each other and are separated by about 1840 bp. Available sequence data and protein analyses [Overbeeke et al., J. Mol. Biol. 163 (1983) 513-522, and this paper] indicate the presence, between gpt and phoE, of two additional genes. These genes are oriented the same way as gpt and code for proteins of 49 and 15.7 kDal, respectively. By in vitro transcription with E. coli RNA polymerase and nuclease S1 analysis, we have identified a promoter upstream of gpt. The short intercistronic region between gpt and the 49-kDal protein gene contains a rho-independent termination signal that closely precedes and partially overlaps another promoter. It appears from these data that gpt transcription is essentially monocistronic, giving rise to RNA of approx. 555 nucleotides, whereas the 49-kDal and 15.7-kDal protein genes are transcribed from their own promoter.