Adaptation of sucrose metabolism in the Escherichia coli wild-type strain EC3132.

Although Escherichia coli strain EC3132 possesses a chromosomally encoded sucrose metabolic pathway, its growth on low sucrose concentrations (5 mM) is unusually slow, with a doubling time of 20 h. In this report we describe the subcloning and further characterization of the corresponding csc genes and adjacent genes. The csc regulon comprises three genes for a sucrose permease, a fructokinase, and a sucrose hydrolase (genes cscB, cscK, and cscA, respectively). The genes are arranged in two operons and are negatively controlled at the transcriptional level by the repressor CscR. Furthermore, csc gene expression was found to be cyclic AMP-CrpA dependent. A comparison of the genomic sequences of the E. coli strains EC3132, K-12, and O157:H7 in addition to Salmonella enterica serovar Typhimurium LT2 revealed that the csc genes are located in a hot spot region for chromosomal rearrangements in enteric bacteria. The comparison further indicated that the csc genes might have been transferred relatively recently to the E. coli wild-type EC3132 at around the time when the different strains of the enteric bacteria diverged. We found evidence that a mobile genetic element, which used the gene argW for site-specific integration into the chromosome, was probably involved in this horizontal gene transfer and that the csc genes are still in the process of optimal adaptation to the new host. Selection for such adaptational mutants growing faster on low sucrose concentrations gave three different classes of mutants. One class comprised cscR(Con) mutations that expressed all csc genes constitutively. The second class constituted a cscKo operator mutation, which became inducible for csc gene expression at low sucrose concentrations. The third class was found to be a mutation in the sucrose permease that caused an increase in transport activity.