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J. Bacteriol., 08 1995, 4742-4747, Vol 177, No. 16
Copyright © 1995, American Society for Microbiology

Characterization of the exbBD operon of Escherichia coli and the role of ExbB and ExbD in TonB function and stability

BM Ahmer, MG Thomas, RA Larsen and K Postle
Department of Genetics and Cell Biology, Washington State University, Pullman 99164-4233, USA.

TonB protein appears to couple the electrochemical potential of the cytoplasmic membrane to active transport across the essentially unenergized outer membrane of gram-negative bacteria. ExbB protein has been identified as an auxiliary protein in this process. In this paper we show that ExbD protein, encoded by an adjacent gene in the exb cluster at 65', was also required for TonB-dependent energy transduction and, like ExbB, was required for the stability of TonB. The phenotypes of exbB exbD+ strains were essentially indistinguishable from the phenotypes of exbB+ exbD strains. Mutations in either gene resulted in the degradation of TonB protein and in decreased, but not entirely absent, sensitivities to colicins B and Ia and to bacteriophage phi 80. Evidence that the absence of ExbB or ExbD differentially affected the half-lives of newly synthesized and steady- state TonB was obtained. In the absence of ExbB or ExbD, newly synthesized TonB was degraded with a half-life of 5 to 10 min, while the half-life of TonB under steady-state conditions was significantly longer, approximately 30 min. These results were consistent with the idea that ExbB and ExbD play roles in the assembly of TonB into an energy-transducing complex. While interaction between TonB and ExbD was suggested by the effect of ExbD on TonB stability, interaction of ExbD with TonB was detected by neither in vivo cross-linking assays nor genetic tests for competition.(ABSTRACT TRUNCATED AT 250 WORDS)


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