Mutational analysis of the Escherichia coli serB promoter region reveals transcriptional linkage to a downstream gene

Abstract

Genes encoding proteins with unrelated functions can be cotranscribed, and this may be used by cells to coordinate different metabolic pathways during growth. We describe a gene, designated sms, which is downstream from the serine biosynthetic gene serB in Escherichia coli but does not appear to be involved in amino acid (aa) biosynthesis. The sms gene is 1380 bp long. The Sms product migrates at 55 kDa on sodium dodecyl sulfate(SDS)-polyacrylamide gels and has a M(r) of 49472 (460 aa residues) calculated from the nucleotide sequence. The deduced Sms aa sequence shares regions of similarity with two ATP-dependent proteases, Lon and RecA, and contains two motifs: a C-x(2)-C-x(n)-C-x(2)-C motif, which is found in some nucleic acid binding proteins, and an ATP/GTP binding site motif. Insertional inactivation of sms led to increased sensitivity to the alkylating agent methylmethane sulfonate, but not to a requirement for serine or other metabolites. Several promoter mutations were isolated and characterized, which suggest that serB has a typical promoter recognized by sigma 70. After the serB coding sequence there is a 48-bp region with no obvious promoter sequence preceding the sms translation start codon. Analyses using sms'-lacZ fusions cloned downstream from wild-type and mutant serB promoters showed that sms is cotranscribed with serB.