Separate regulation of purA and purB loci of Escherichia coli K-12

Abstract

We isolated a strain of Escherichia coli K-12 in which the lac structural genes are fused to the purB control region and used this strain to study the regulation of the purA and purB loci. The purA locus was derepressed in response to either limiting adenine or guanine growth conditions in the presence of excess guanine or adenine, respectively. The presence of hypoxanthine in the culture medium did not have any effect on the expression of the purA locus. The purB locus responded to limiting adenine growth conditions in the presence of either excess hypoxanthine or guanine alone but not when both hypoxanthine and guanine were present. Adenylosuccinate (SAMP) lyase (EC 4.3.2.2), the product of the purB locus in Escherichia coli K-12 and Salmonella typhimurium, is a bifunctional enzyme that catalyzes the conversion of phosphoribosylsuccinocarboxamide aminoimidazole (SAICAR) to phosphoribosylaminoimidazolecarboxarnide and SAMP to AMP (8; Fig. 1). Strains with a purB mutation exhibit a specific requirement for adenine and accumulate SAICAR. SAMP synthetase (EC 6.3.4.4), the product of the purA locus, and SAMP lyase form a two-step pathway for AMP biosynthesis via SAMP from IMP (Fig. 1).SAMP synthetase and SAMP lyase have been physically characterized from E. coli (16,19,22) and several other sources. In E. coli both enzyme activities are known to be repressed when adenine is added to culture media, but the exact mechanisms of regulation and the number of regulatory loci involved are not known (3, 7). Since the purA and purB loci are unlinked on the E. coli K-12 chromosome (1) and the purB locus is also involved in IMP biosynthesis, the two loci would not a priori be expected to be regulated by

Purine biosynthesis in Escherichia coli K12: structure and DNA sequence studies of the purHD locus

“…Strain BSJ72 (obtained from Tom St. John, Fred Hutchinson Cancer Research Center) was used for the lacZ^ complementation inactivation identification of subclones and the production of sequencing DNA by superinfection with the M13 helper phage K07 (Vieira and Messing, 1987). Strain TX302 (Wolfe and Smith, 1985) is our standard strain for genetic and regulatory studies. Strains were made competent and transformed as described (Alexander et al, 1984).…”