Growth of Azospirillum irakense KBC1 on the Aryl beta-glucoside salicin requires either salA or salB

The rhizosphere nitrogen-fixing bacterium Azospirillum irakense KBC1 is abl e to grow on pectin and beta-glucosides such as cellobiose, arbutin, and sa licin, Two adjacent genes, salA and salB, conferring beta-glucosidase activ ity to Escherichia coli, have been identified in a cosmid library of A. ira kense DNA, The SalA and SalB enzymes preferentially hydrolyzed aryl beta-gl ucosides. A Delta(salA-salB) A. irakense mutant was not able to grow on sal icin but could still utilize arbutin, cellobiose, and glucose for growth. T his mutant could be complemented by either salA or salB, suggesting functio nal redundancy of these genes in salicin utilization. In contrast to this f unctional homology, the SalA and SalB proteins, members of family 3 of the glycosyl hydrolases, show a low degree of amino acid similarity. Unlike Sal A, the SalB protein exhibits an atypical truncated C-terminal region. We pr opose that SalA and SalB are representatives of the AB and AB' subfamilies, respectively, in glycosyl hydrolase family 3, This is the first genetic im plication of this beta-glucosidase family in the utilization of beta-glucos ides for microbial growth.