Identification of a Cellobiose Utilization Gene Cluster with Cryptic β-Galactosidase Activity in Vibrio fischeri

Abstract

ABSTRACT Cellobiose utilization is a variable trait that is often used to differentiate members of the family Vibrionaceae . We investigated how Vibrio fischeri ES114 utilizes cellobiose and found a cluster of genes required for growth on this β-1,4-linked glucose disaccharide. This cluster includes genes annotated as a phosphotransferase system II ( celA , celB , and celC ), a glucokinase ( celK ), and a glucosidase ( celG ). Directly downstream of celCBGKA is celI , which encodes a LacI family regulator that represses cel transcription in the absence of cellobiose. When the celCBGKAI gene cluster was transferred to cellobiose-negative strains of Vibrio and Photobacterium , the cluster conferred the ability to utilize cellobiose. Genomic analyses of naturally cellobiose-positive Vibrio species revealed that V. salmonicida has a homolog of the celCBGKAI cluster, but V. vulnificus does not. Moreover, bioinformatic analyses revealed that CelG and CelK share the greatest homology with glucosidases and glucokinases in the phylum Firmicutes . These observations suggest that distinct genes for cellobiose utilization have been acquired by different lineages within the family Vibrionaceae . In addition, the loss of the celI regulator, but not the structural genes, attenuated the ability of V. fischeri to compete for colonization of its natural host, Euprymna scolopes , suggesting that repression of the cel gene cluster is important in this symbiosis. Finally, we show that the V. fischeri cellobioase (CelG) preferentially cleaves β- d -glucose linkages but also cleaves β- d -galactose-linked substrates such as 5-bromo-4-chloro-3-indolyl-β- d -galactoside (X-gal), a finding that has important implications for the use of lacZ as a marker or reporter gene in V. fischeri .