The salCAB operon of Azospirillum irakense, required for growth on salicin, is repressed by SalR, a transcriptional regulator that belongs to the LacI/GalR family

The salAB genes of Azospirillum irakense KBC1, which encode two aryl-beta-g lucosidases, are required for growth on salicin. In the 4-kb region upstrea m of the salAB genes, two additional genes, salC and salR, were identified. SalC shows characteristics of the outer membrane receptors in the FepA/Fhu A family. The salCAB genes are transcribed as a polycistronic mRNA. The sal R gene encodes a protein homologous to the LacI/GalR family of transcriptio nal repressors. Expression of the sal operon, measured by means of a salC-g usA translational fusion in A. irakense KBC1, requires the presence of aryl -beta-glucosides such as arbutin and salicin. Expression is markedly enhanc ed when a simple carbon source, like glucose, cellobiose or malate, is adde d to the medium. In a salR mutant, expression of the salC-gusA fusion does not require an aryl-beta-glucoside inducer. Expression of a sal-R-gusA fusi on is constitutive. The product of arbutin hydrolysis (hydroquinone) partly inhibits the expression of a salC-gusA fusion in arbutin- or salicin-conta ining minimal medium. This effect is independent of SalR. Salicylalcohol, t he hydrolysis product of salicin, also partly inhibits salC expression in a SalR-independent fashion, but only in salicin-containing minimal medium.