Analysis of the beta-glucoside utilization (bgl) genes of Shigella sonnei:evolutionary implications for their maintenance in a cryptic state

The pattern of expression of the genes involved in the utilization of aryl beta-glucosides such as arbutin and salicin is different in the genus Shige lla compared to Escherichia coli. The results presented here indicate that the homologue of the cryptic bgl operon of E. coli is conserved in Shigella sonnei and is the primary system involved in beta-glucoside utilization in the organism. The organization of the bgl genes in 5. sonnei is similar to that of E. coli; however there are three major differences in terms of the ir pattern of expression. (i) The bglB gene, encoding phospho-beta-glucosid ase B, is insertionally inactivated in 5. sonnei. As a result, mutational a ctivation of the silent bgl promoter confers an Arbutin-positive (Arb(+)) p henotype to the cells in a single step; however, acquiring a Salicin-positi ve (Sal(+)) phenotype requires the reversion or suppression of the bglB mut ation in addition. (ii) Unlike in E. coli, a majority of the activating mut ations (conferring the Arb(+) phenotype) map within the unlinked hns locus, whereas activation of the E. coli bgl operon under the same conditions is predominantly due to insertions within the bglR locus. (iii) Although the b gl promoter is silent in the wild-type strain of 5. sonnei (as in the case of E. coli), transcriptional and functional analyses indicated a higher bas al level of transcription of the downstream genes. This was correlated with a 1 bp deletion within the putative Rho-independent terminator present in the leader sequence preceding the homologue of the bglG gene. The possible evolutionary implications of these differences for the maintenance of the g enes in the cryptic state are discussed.