THE MUTL REPAIR GENE OF ESCHERICHIA-COLI K-12 FORMS A SUPEROPERON WITH A GENE ENCODING A NEW CELL-WALL AMIDASE

We report a molecular genetic analysis of the region immediately upstr eam from the Escherichia cell mutL DNA repair gene at 94.8 min. An ope n reading frame ending 9 bp upstream from the start of mutL correspond s to a 48 kDa polypeptide detected previously in minicells. The predic ted amino acid sequence of this 48 kDa polypeptide shows homology to t he major N-acetylmuramoyl-L-alanine amidase autolysin of Bacillus subt ilis, a known amidase of Bacillus licheniformis, and the product of a Salmonella typhimurium gene that maps near 50 min. Insertions in this upstream gene, which we named amiB, or in mutL did not affect cell sha pe or viability; however, overexpression of the Amis polypeptide cause d cell lysis, hypersensitivity to osmotic shock and treatment with wat er, and temporary autolysis by low levels of antibiotics, which are al l consistent with Amis acting as a cell-wall hydrolase. Analysis of ch romosomal transcription demonstrated that amiB forms a complex operon with mutL and two additional upstream genes. mutL transcripts also ori ginated from an internal promoter, designated P-mutL, located in amiB 312 bp upstream from the translational start of mutL. Together, these results suggest that E. coli contains a second amidase possibly involv ed in cell-wall hydrolysis, septation, or recycling, and that transcri ption of this amidase is directly linked to a gene central for DNA rep air.