The source of energy for bacterial motility is the intermediate in In eukaryotes the transduction of chemical into mechanical energy (movement) involves ATP whenever it has been determined. Thus muscles contract and cilia and flagella beat by a sliding filament mechanism that uses ATP directly (1-3).In prokaryotes the immediate source of energy used in chemomechanical coupling is unknown.Cells can generate ATP either by oxidative phosphorylation, resulting from the oxidation of substrates via electron transport, or by substrate-level phosphorylation, even anaerobically as in glycolysis. The

The nucleotide sequence of the Escherichia coli K12 beta-methylgalactoside transport operon, mgl, was determined. Primer extension analysis indicated that the synthesis of mRNA initiates at guanine residue 145 of the determined sequence. The operon contains three open reading frames (ORF). The operator proximal ORF, mglB, encodes the galactose binding protein, a periplasmic protein of 332 amino acids including the 23 residue amino-terminal signal peptide. Following a 62 nucleotide spacer, the second ORF, mglA, is capable of encoding a protein of 506 amino acids. The amino-terminal and carboxyl-terminal halves of this protein are homologous to each other and each half contains a putative nucleotide binding site. The third ORF, mglC, is capable of encoding a hydrophobic protein of 336 amino acids which is thought to generate the transmembrane pore. The overall organization of the mglBAC operon and its potential to encode three proteins is similar to that of the ara FGH high affinity transport operon, located approximately 1 min away on the E. coli K12 chromosome.

Expression plasmids containing various portions of araFGH operon sequences were assayed for their ability to facilitate the high-affinity L-arabinose transport process in a strain lacking the chromosomal copy of this operon. Accumulation studies demonstrated that the specific induction of all three operon coding sequences was necessary to restore high-affinity L-arabinose transport. Kinetic analysis of this genetically reconstituted transport system indicated that it functions with essentially wild-type parameters. Therefore, L-arabinosebinding protein-mediated transport appears to require only two inducible membrane-associated components (araG and araH) in addition to the binding protein (araF).The active accumulation of L-arabinose by the enteric bacterium Escherichia coli is mediated by two distinct systems. The first of these systems was reported by D. Isaacson and E. Englesberg (Bacteriol. Proc. 1964:113-114) as an inducible, energy-dependent accumulation process with a Km of entry (Ken) of initial L-arabinose uptake of 1.25 X 10-4 M. Mutations causing defects in this sugar-ion symport system mapped to a single locus, designated araE, at 60 min on the E. coli chromosome (13,19). The araE gene product has been identified as a 52,000-dalton, membraneassociated protein (15,16). The second L-arabinose transport system has a greater affinity for L-arabinose, exhibiting a Ken of 1 x 10-6 to 3 x 10-6 M (3, 21). This high-affinity transport system has been shown to be dependent on the activity of the L-arabinose-binding protein (3,4).The locus encoding the L-arabinose-binding protein, araF, is part of an operon located at 45 min on the E. coli chromosome (3, 4). Operon sequence analysis and expression studies indicate that the promoter-proximal bindingprotein-coding sequence (araF) is followed by two additional loci designated araG and araH (13,22). As expected, the L-arabinose-binding protein is encoded by araF and is localized in the periplasm of the cell, whereas araG and araH encode two membrane-associated proteins (52,000 and 31,000 daltons, respectively) (12).Analyses of the deduced amino acid sequences of the araG and araH proteins reveal significant homologies with membrane components of a number of periplasmic bindingprotein-mediated transport systems (22). The araG gene product has been shown to be homologous to the hisP, pstB, malK, and rbsA gene products, all of which presumably play similar roles in the histidine, phosphate, maltose, and ribose transport systems, respectively. These regions of homology also contain a putative ATP-binding site, implicating this group of proteins in the coupling of cellular energy to the transport process. The araH gene product is less homologous with components of the other systems examined, with the exception of the rbsC protein of the ribose system. However, the extremely hydrophobic character of the araH product is shared with many components of other periplasmic permeases (22).A survey of periplasmic binding-protein-mediated transport systems indicates that the nu...