As. Kharat et S. Mahadevan, Analysis of the beta-glucoside utilization (bgl) genes of Shigella sonnei:evolutionary implications for their maintenance in a cryptic state, MICROBIO-UK, 146, 2000, pp. 2039-2049
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.