Using degenerate oligodeoxyribonucleotides from conserved regions of the gene family encoding ATP-binding domain of the active transporter, two new Escherichia coli genes were identified. The first of the genes, named mdl (multidrug resistance-like), is located at min 10.2 of the E. coli chromosome and encodes two ATP-binding motifs and two hydrophobic (transmembrane) domains. The ATP-binding domains of mdl show 35-38% amino acid (aa) identity with members of the eukaryotic P-glycoprotein/multidrug resistance family. To date, 25 members of the ATP-transporter/permease gene family have been characterized in E. coli. Comparison of the ATP-binding domains from this family indicates that mdl is part of a distinct subfamily of sequences that includes hlyB, msbA, and cvaB. Gene-disruption studies revealed that mdl is not essential for cell growth. The second open reading frame, named abc (ATP-binding cassette), is located at min 4.9 of the chromosome, encodes a single ATP-binding domain, and is most homologous to ftsE, a cell division control gene of E. coli. The abc gene product also shows aa sequence homology to several E. coli permeases.