Diverse pathways for salicin utilization in Shigella sonnei and Escherichia coli carrying an impaired bgl operon
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ORIGINAL PAPER
Diverse pathways for salicin utilization in Shigella sonnei and Escherichia coli carrying an impaired bgl operon Stuti K. Desai · Krithi Nandimath · S. Mahadevan
Received: 17 May 2010 / Revised: 19 July 2010 / Accepted: 26 July 2010 / Published online: 10 August 2010 © Springer-Verlag 2010
Abstract Utilization of the aryl--glucosides salicin or arbutin in most wild-type strains of E. coli is achieved by a single-step mutational activation of the bgl operon. Shigella sonnei, a branch of the diverse E. coli strain tree, requires two sequential mutational steps for achieving salicin utilization as the bglB gene, encoding the phospho--glucosidase B, harbors an inactivating insertion. We show that in a natural isolate of S. sonnei, transcriptional activation of the gene SSO1595, encoding a phospho--glucosidase, enables salicin utilization with the permease function being provided by the activated bgl operon. SSO1595 is absent in most commensal strains of E. coli, but is present in extraintestinal pathogens as bgcA, a component of the bgc operon that enables -glucoside utilization at low temperature. Salicin utilization in an E. coli bglB laboratory strain also requires a two-step activation process leading to expression of BglF, the PTS-associated permease encoded by the bgl operon and AscB, the phospho--glucosidase B encoded by the silent asc operon. BglF function is needed since AscF is unable to transport -glucosides as it lacks the IIA domain involved in phopho-relay. Activation of the asc operon in the Sal+ mutant is by a promoter-up mutation and the activated operon is subject to induction. The pathway to achieve salicin utilization is therefore diverse in these two evolutionarily related organisms; however, both show cooperation between two silent genetic systems to achieve a new metabolic capability under selection.
Communicated by Jan Roelof van der Meer. S. K. Desai · K. Nandimath · S. Mahadevan (&) Department of Molecular Reproduction, Development and Genetics, Indian Institute of Science, Bangalore 560012, India e-mail: [email protected]
Keywords Silent genes · Beta-glucosides · Transcriptional activation
Introduction Decaying plant matter contributes substantially to the richness of soil in terms of the carbon sources it provides to the residing microbes. Humus often contains plant-derived -glucosides in which there is a (1–4) linkage between glucose and an aryl or an alkyl group. The two important steps for the utilization of aryl--glucosides, arbutin and salicin, by bacteria are their transport inside the cell and subsequent hydrolysis by enzymes belonging to the class of phospho--glucosidases. Aromatic -glucosides are transported by permeases belonging to the phosphoenolpyruvate—phosphotransferase system (PTS) and converted to their 6-phosphoryl derivatives. The phosphorylated forms are then cleaved by phospho--glucosidases to yield glucose-6-phosphate and an aglycone (Fox and Wilson 1968). In the family Enterobacteriacea, the pattern of aryl-glucoside utilization va
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