Truncated betB2-144 plays a critical role in Sinorhizobium meliloti Rm2011 osmoprotection and glycine-betaine catabolism

Abstract Sinorhizobium meliloti salt tolerance is important in establishing a nitrogen fixing symbiosis with alfalfa under high osmolarity. In order to resist osmotic stress, rhizobia accumulate a number of osmolytes including a potent osmoprotectant, glycine-betaine (GB). S. meliloti is able to synthesize GB from choline via a pathway encoded by the chromosomal betICBA operon. The last enzyme in the biochemical pathway, BetB, encodes a betaine aldehyde dehydrogenase, which converts betaine aldehyde to GB. Deletion of betB in S. meliloti strain Rm2011 leads to loss of detectable BADH activity. A second betB -related gene, betB2 , is present on the S. meliloti pSymA megaplasmid and deletion of this gene has no effect on the observed level of BADH. However, an Rm2011 deletion mutant with only 144 aa from the N-terminal domain of BetB2 has lost the ability to tolerate high osmolarity, no longer accumulates GB under osmotic stress, has high levels of NAD-BADH, and has gained the ability to utilize GB for growth. These results suggest that truncated BetB2-144 stimulates GB catabolism, preventing it from being used as an osmoprotectant.