A Rhizobium meliloti homolog of the Escherichia coli peptide-antibiotic transport protein SbmA is essential for bacteroid development.

Alfalfa nodules induced by a Rhizobium meliloti strain carrying the bacA386::TnphoA mutation (formerly fix386::TnphoA) were examined by light and electron microscopy. These ineffective nodules were found to contain bacteria within infection threads, but no mature bacteroids were observed. A closer examination revealed that there were undeveloped senescent bacteroids in the plant cells of the nodule invasion zone, strongly suggesting that the symbiotic defect of the bacA386::TnphoA mutant is attributable to an early block in bacteroid development. The expression of the bacA gene in effective nodules was monitored with a bacA-phoA fusion and found to be strongest in the region where developing bacteroids are found. The bacA+ gene was cloned and sequenced. Sequence analysis indicated that BacA is probably an integral inner membrane protein with seven transmembrane domains and that it is extremely homologous to Escherichia coli SbmA, an inner membrane protein required for the uptake of microcin B17, a peptide antibiotic. Southern blotting experiments indicate that a gene closely related to bacA/sbmA is found in many bacteria, including some that invade eukaryotic cells. Possible roles for BacA in symbiosis are discussed.