Genetic analysis of the Sinorhizobium meliloti BacA protein: differential effects of mutations on phenotypes

Sinorhizobium meliloti strains lacking BacA function are impaired in symbiosis with alfalfa host plants and display altered sensitivities to a number of compounds relative to wild-type strains. With the goal of finding clues to the currently unknown biological function(s) of BacA, we carried out a genetic analysis to determine which amino acids are critical for protein function and to attempt to ascertain whether the multiple phenotypes that result from a bacA-null allele were the result of a common cause or whether BacA has multiple functions. We have created a set of 20 site-directed mutants in which selected individual amino acids in bacA were replaced with glycine residues. The resulting mutants were characterized to determine how the various amino acid changes affected a number of phenotypes associated with loss of BacA function. Mutants H165G, W182G, D198G, and R284G had null phenotypes for all functions assayed, while mutants W57G, S83G, S231G, and K350G were indistinguishable from wild-type strains. The remaining 12 site-directed mutants demonstrate mixed phenotypic characteristics and fall into a number of distinctly different groups. These observations may be consistent with a role for BacA in multiple, nonoverlapping functions.