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J Bacteriol, May 1998, p. 2623-2629, Vol. 180, No. 10
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

The L-Isoaspartyl Protein Repair Methyltransferase Enhances Survival of Aging Escherichia coli Subjected to Secondary Environmental Stresses

Jonathan E. Visick, Hui Cai, and Steven Clarke*

Department of Chemistry and Biochemistry and the Molecular Biology Institute, University of California, Los Angeles, California 90095-1569

Received 28 January 1998/Accepted 11 March 1998

Like its homologs throughout the biological world, the L-isoaspartyl protein repair methyltransferase of Escherichia coli, encoded by the pcm gene, can convert abnormal L-isoaspartyl residues in proteins (which form spontaneously from asparaginyl or aspartyl residues) to normal aspartyl residues. Mutations in pcm were reported to greatly reduce survival in stationary phase and when cells were subjected to heat or osmotic 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-specific sigma factor (J. E. Visick and S. Clarke, J. Bacteriol. 179:4158-4163, 1997). We now show that the rpoS mutation, resulting in a 90% decrease in HPII catalase activity, can account for the previously observed phenotypes. We further demonstrate that a new pcm mutant lacks these phenotypes. Interestingly, the newly constructed pcm mutant, when maintained in stationary phase for extended periods, is susceptible to environmental stresses, including exposure to methanol, oxygen radical generation by paraquat, high salt concentrations, and repeated heating to 42°C. The pcm mutation also results in a competitive disadvantage in stationary-phase cells. All of these phenotypes can be complemented by a functional pcm gene integrated elsewhere in the chromosome. These data suggest that protein denaturation and isoaspartyl formation may act synergistically to the detriment of aging E. coli and that the repair methyltransferase can play a role in limiting the accumulation of the potentially disruptive isoaspartyl residues in vivo.

* Corresponding author. Mailing address: Dept. of Chemistry and Biochemistry, Box 951569, Los Angeles, CA 90095-1569. Phone: (310) 825-8754. Fax: (310) 825-1968. E-mail: clarke{at}ewald.mbi.ucla.edu.

J Bacteriol, May 1998, p. 2623-2629, Vol. 180, No. 10
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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