A 3.6-kb DNA fragment from Bacillus subtilis was found to complement the K+ uptake-deficient Escherichia coli strain TK2420. Transformation with a pKLO61 plasmid harboring this fragment conferred the capacity to grow on a minimal medium containing only 10 mM K+. Insertional mutagenesis and subcloning identified a single gene responsible for the complementation. This gene coded for an apparent homolog of E. coli TrkA. Sequence analysis of the cloned region also revealed three additional open reading frames. These included: a gene encoding a homolog to the czcD gene product of Alcaligenes eutrophus, a lysR-type regulatory gene which was found to enhance Na+ resistance in E. coli NM81 (delta nhaA) in a separate complementation test, and an orfD with no significant similarity to sequences deposited in Genbank.