THE L-ISOASPARTYL PROTEIN REPAIR METHYLTRANSFERASE ENHANCES SURVIVAL OF AGING ESCHERICHIA-COLI SUBJECTED TO SECONDARY ENVIRONMENTAL STRESSES

Like its homologs throughout the biological world, the L-isoaspartyl p rotein repair methyltransferase of Escherichia coli, encoded by the pc m gene, can convert abnormal L-isoaspartyl residues in proteins (which form spontaneously from asparaginyl or aspartyl residues) to normal a spartyl residues. Mutations in pcm were reported to greatly reduce sur vival in stationary phase and when cells were subjected to heat or osm otic stresses (C. Li and S. Clarke, Proc. Natl, Acad, Sci, USA 89:9885 -9889, 1992), However, we subsequently demonstrated that those strains had a secondary mutation in rpoS, which encodes a stationary-phase-sp ecific sigma factor (J. E. Visick and S. Clarke, J. Bacteriol. 179:415 8-4163, 1997), We now show that the rpoS mutation, resulting in a 90% decrease in HPII catalase activity, can account for the previously obs erved phenotypes, We further demonstrate that a new pcm mutant lacks t hese phenotypes. Interestingly, the newly constructed pcm mutant, when maintained in stationary phase for extended periods, is susceptible t o environmental stresses, including exposure to methanol, oxygen radic al generation by paraquat, high salt concentrations, and repeated heat ing to 42 degrees C. The pcm mutation also results in a competitive di sadvantage in stationary-phase cells. All of these phenotypes can be c omplemented by a functional pcm gene integrated elsewhere in the chrom osome. These data suggest that protein denaturation and isoaspartyl fo rmation may act synergistically to the detriment of aging E. coli and that the repair methyltransferase can play a role in limiting the accu mulation of the potentially disruptive isoaspartyl residues in vivo.