Growth of Azospirillum irakense KBC1 on the Aryl β-Glucoside Salicin Requires either salA or salB

The rhizosphere nitrogen-fixing bacterium Azospirillum irakense KBC1 is able to grow on pectin and β-glucosides such as cellobiose, arbutin, and salicin. Two adjacent genes, salA and salB, conferring β-glucosidase activity to Escherichia coli, have been identified in a cosmid library of A. irakense DNA. The SalA and SalB enzymes preferentially hydrolyzed aryl β-glucosides. A Δ(salA-salB) A. irakense mutant was not able to grow on salicin but could still utilize arbutin, cellobiose, and glucose for growth. This mutant could be complemented by either salA or salB, suggesting functional redundancy of these genes in salicin utilization. In contrast to this functional homology, the SalA and SalB proteins, members of family 3 of the glycosyl hydrolases, show a low degree of amino acid similarity. Unlike SalA, the SalB protein exhibits an atypical truncated C-terminal region. We propose that SalA and SalB are representatives of the AB and AB′ subfamilies, respectively, in glycosyl hydrolase family 3. This is the first genetic implication of this β-glucosidase family in the utilization of β-glucosides for microbial growth.