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

Abstract

ABSTRACT 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.