Tricarboxylate transport in a Cit+ Escherichia coli: evidence for the role of an outer membrane protein.

A strain of Escherichia coli of bovine origin able to use tricarboxylates as single carbon source is described. Tricarboxylate utilization (Cit+) and fluorocitrate sensitivity (FCs) could be transferred conjugatively to E. coli K12 and were not plasmid borne. No evidence was found for tct gene products of Salmonella typhimurium. A citrate-inducible outer membrane protein of 21-22 kilodaltons (kd) was found only in Cit+ strains. A protein (21-22kd) protein was also found in wild-type E. coli K12 and in fluorocitrate-resistant mutants of Cit+ strains, but it was present in a cryptic form no longer inducible by citrate. Fluorocitrate-resistant mutants of Cit+ strains were still able to transport citrate by a fluorocitrate-insensitive system. High levels of the 22-kd protein correlated with reduced growth induction times on citrate and with the ability to effectively transport citrate.