Overproduction of penicillin-binding protein 7 suppresses thermosensitive growth defect at low osmolarity due to an spr mutation of Escherichia coli.

Escherichia coli delta prc mutants lacking periplasmic protease Prc, which was originally found involved in the C-terminal processing of penicillin-binding protein (PBP) 3, show thermosensitive growth at low osmolarity. We isolated thermoresistant revertants containing extragenic suppressor (spr) mutations. In the prc+ background the mutations also caused thermosensitivity at low osmolarity. They were all mapped at about 48 min on the chromosome and most probably allelic to one another. From this chromosomal region we cloned a gene that could correct the thermosensitive defect of an spr mutant, which turned out to be a multicopy suppressor of spr. Analysis of the nucleotide sequence predicted that the gene would code for a low-molecular-weight PBP, and penicillin-binding experiments revealed the product to be PBP 7. Disruption of the gene on the chromosome caused no apparent growth defect. PBP 7 seemed to be degraded by protease Prc. Overproduction of mutant PBP 7 that had the active site serine residue replaced with alanine did not correct the spr thermosensitivity, suggesting importance of the DD-endopeptidase activity in the multicopy suppression.