Genes involved in low-affinity transport of D-ribose in Escherichia coli k-12

Abstract

Mutants defective in the high-affinity transport of D-ribose are able to utilize ribose as a sole carbon and energy source, suggesting that other low-affinity transport systems for the sugar exist in the cells. In order to search for these transport systems, transposon mutagenesis was performed. Mutants with different phenotypes on ribose minimal growth were observed and classified into five groups. The precise locations of insertions on the chromosome were determined by in vivo cloning and analysis using polA mutation. Group I insertions revealed an operon encoding a new ABC-type transporter, located in 92.8 min. Group II insertions lie in rbs operon with defective growth on ribose due to a polar inactivation of rbsK, the gene for ribokinase. Group III insertion was found in the open reading frame, f547 coding for a potential integral membrane protein similiar to the proton symporter XylE. Insertions in xylA and its promoter region (Group IV) enhanced the growth on ribose, which was abolished by secondary insertion (Group V) in xylG gene that is divergently transcribed from xylA. XylG protein appears to be a component of the high affinity transporter for D-xylose. This indicates that the high affinity D-xylose transport system also supports the uptake of D-ribose.