| dc.description.abstract | Genetic Organization and Regulation of -Glucoside Utilization Genes in Klebsiella aerogenes
Abstract and Synopsis
Introduction
The plant-derived aromatic -glucosides salicin (Sal) and arbutin (Arb), along with cellobiose (Cel), a degradation product of cellulose and lichenin, are common naturally occurring -glucosides. Bacteria utilize these sugars via specific transport proteins that phosphorylate the C-6 hydroxyl group, followed by cleavage of the glycosidic bond by phospho- -glucosidases, releasing glucose-6-phosphate for glycolysis.
Within the Enterobacteriaceae, variations exist in the ability to metabolize these glucosides. For example, wild-type strains of the Klebsiella-Aerobacter group ferment salicin, arbutin, and cellobiose, whereas most Citrobacter and Salmonella strains show weak or no metabolism. In Escherichia coli, the bgl operon is cryptic and activated only after mutational events. The regulation and functional status of homologous systems in other organisms remain poorly understood.
Key Findings
Enzyme Activity: Basal -glucosidase activity in Klebsiella was 3-6 fold higher than in E. coli strains carrying activated bgl alleles. Activity was inducible by salicin and cellobiose, suggesting multiple systems.
Mutant Analysis: EMS-induced Sal mutants were Arb but remained Cel , indicating at least two independent systems for -glucoside utilization. Mutants showed compromised induction, pointing to regulatory gene involvement.
Gene Identification: Southern blotting and BLAST analysis confirmed homologues of E. coli bgl genes in K. aerogenes. The cloned Ka bglB complemented E. coli mutants and carried conserved catalytic motifs. Unlike K. oxytoca CasA, Ka BglB could not hydrolyze cellobiose, confirming substrate specificity differences.
Regulatory Genes: The cloned Ka bglG complemented E. coli mutants and encoded a protein homologous to the BglG/SacY family of antiterminators, with conserved PRD domains and phosphorylation sites.
Regulatory Sequences: Negative elements silencing E. coli bgl genes (dyad symmetry and H-NS binding sites) were absent in K. aerogenes, explaining higher basal activity.
Promoter Organization: Multiple promoters were identified, suggesting differential regulation under varying conditions.
Regulation by H-NS and Glucose: H-NS showed a stimulatory role in K. aerogenes bgl transcription, unlike its repressive role in E. coli. Glucose repressed bgl expression only two-fold, compared to drastic repression in E. coli.
Conclusions
The bgl genes of K. aerogenes operate similarly to those in E. coli but with notable differences:
Higher basal -glucosidase activity.
Absence of strong negative regulatory elements.
Substrate specificity restricted to aryl- -glucosides.
Flexible promoter organization allowing modulation under environmental cues.
This study illustrates how subtle molecular variations enable organisms to optimize resource utilization in their ecological niches. | |
