Cloning of mglB, the structural gene for the galactose-binding protein of Salmonella typhimurium and Escherichia coli

Cloning and DNA sequence analysis of an immunogenic glucose-galactose MglB lipoprotein homologue from Brachyspira pilosicoli, the agent of colonic spirochetosis

Colonic spirochetosis (CS) is a newly emerging infectious disease of humans and animals caused by the pathogenic spirochete Brachyspira (formerly Serpulina) pilosicoli. The purpose of this study was to characterize an antigen that was recognized by antibodies present in sera of challenge-exposed pigs. The gene encoding the antigen was identified by screening a plasmid library of human B. pilosicoli strain SP16 (ATCC 49776) genomic DNA with hyperimmune and convalescent swine sera. The predicted amino acid sequence encoded by the cloned B. pilosicoli gene had a high degree of similarity and identity to glucose-galactose MglB lipoprotein. Located 106 bp downstream of the putative mglB gene was a 3'-truncated open reading frame with 73.8% similarity and 66.3% identity to mglA of Escherichia coli, suggesting a gene arrangement within an operon which is similar to those of other bacteria. A single copy of the gene was present in B. pilosicoli, and homologous sequences were widely conserved among porcine intestinal spirochetes Serpulina intermedia, Brachyspira innocens, Brachyspira murdochii, and the avian Brachyspira alvinipulli, but not in porcine Brachyspira hyodysenteriae, human Brachyspira aalborgi, and porcine Treponema succinifaciens. The deduced molecular weight of the mature MglB lipoprotein was consistent with expression by the cloned gene of a polypeptide with an apparent molecular weight of 36,000, as determined by Western blot analysis and [(3)H]palmitate labeling. Because mucin is the principal constituent of the colonic mucus gel and consists of glycoproteins that can serve as the substrate for growth and chemotaxis of B. pilosicoli in vitro, a role for MglB in mucosal localization of the spirochete appears consistent with the pathogenesis of CS. However, the presence of homologous sequences in closely related but nonpathogenic commensal spirochetes suggests that other virulence determinants may be required for pathogenesis.

Similarity between the 38-kilodalton lipoprotein of Treponema pallidum and the glucose/galactose-binding (MglB) protein of Escherichia coli

The recent discovery that abundant and immunogenic lipoproteins constitute the integral membrane proteins of Treponema pallidum has prompted efforts to investigate their importance in the physiology and ultrastructure of the organism and in immune responses during infection. Earlier studies identified a 38-kDa lipoprotein of T. pallidum believed to be specific to the pathogen. In the present study, monoclonal antibodies generated against the 38-kDa lipoprotein of T. pallidum reacted with cognate 37-kDa molecules in the nonpathogens Treponema phagedenis, Treponema denticola, and Treponema refringens. Cloning and expression of the 38-kDa-lipoprotein gene of T. pallidum in Escherichia coli revealed that the recombinant product displayed a slightly larger (39-kDa) apparent molecular mass but remained reactive with anti-38-kDa-protein monoclonal antibodies. The recombinant product was processed and acylated in E. coli. DNA and amino acid sequence analyses indicated an open reading frame encoding 403 amino acids, with the first 25 amino acids corresponding to a leader peptide terminated by a signal peptidase II processing site of Val-Val-Gly-Cys. The predicted mature protein is 378 amino acids in length with a deduced molecular weight of 40,422 (excluding acylation). Southern blotting failed to demonstrate in nonpathogenic treponemes genomic sequences homologous with the 38-kDa-lipoprotein gene of T. pallidum. Computer analysis revealed that the 38-kDa lipoprotein of T. pallidum had 34.2% identity and 58.9% similarity with the glucose/galactose-binding protein (MglB) of E. coli and Salmonella typhimurium. Furthermore, of the 19 amino acids of MglB involved in carbohydrate binding, the 38-kDa lipoprotein had identity with 11. These studies have allowed the first putative functional assignment (carbohydrate binding) to a T. pallidum integral membrane protein. Recognition of this potential physiological role for the 38-kDa lipoprotein underscores the possibility that the membrane biology of T. pallidum may more closely resemble that of gram-positive organisms, which also utilize lipoproteins as anchored transporters, than that of gram-negative bacteria to which T. pallidum often is analogized.

Ribose and glucose-galactose receptors. Competitors in bacterial chemotaxis

The periplasmic ribose and glucose-galactose receptors (binding proteins) of Gram-negative bacteria compete for a common inner membrane receptor in bacterial chemotaxis, as well as being the essential primary receptors for their respective membrane transport systems. The high-resolution structures of the periplasmic receptors for ribose (from Escherichia coli) and glucose or galactose (from both Salmonella typhimurium and E. coli) are compared here to outline some features that may be important in their dual functions. The overall structure of each protein consists of two similar domains, both of which are made up of two non-contiguous segments of amino acid chain. Each domain is composed of a core of beta-sheet flanked on both sides with alpha-helices. The two domains are related to each other by an almost perfect intramolecular axis of symmetry. The ribose receptor is smaller as a result of a number of deletions in its sequence relative to the glucose-galactose receptor, mostly occurring in the loop regions; as a result, this protein is also more symmetrical. Many structural features, including some hydrophobic core interactions, a buried aspartate residue and several unusual turns, are conserved between the two proteins. The binding sites for ligand are in similar locations, and built along similar principles, although none of the specific interactions with the sugars is conserved. A comparison shows further that slightly different rotations relate the domains to each other in the three proteins, with the ribose receptor being the most closed, and the Salmonella glucose-galactose receptor the most open. The primary axis of relative rotation is almost perpendicular to that which describes the intramolecular symmetry in each case. These relative rotations of the domains are accompanied by the sliding of some helices as the structures adjust themselves to relieve strain. The hinges which are responsible for most of these relative domain rotations are very similar in the three proteins, consisting of a symmetrical arrangement of beta-strands and alpha-helices and two conserved water molecules that are critical to the hydrogen bonding in the important interdomain region. A region of high sequence and structural similarity between the ribose and glucose-galactose receptors is also located around the intramolecular symmetry axis, on the opposite side of the proteins from the hinge region. This region is that which is altered most by the relative rotations, and is the location of most of the known mutations which affect chemotaxis and transport in the ribose receptor.

The mglB sequence of Salmonella typhimurium LT2; promoter analysis by gene fusions and evidence for a divergently oriented gene coding for the mgl repressor

The mglB gene of Salmonella typhimurium LT2 coding for the galactose-binding protein (GBP) was sequenced. We compared the deduced amino acid sequence with the GBP sequence of Escherichia coli K 12. The mature proteins differ in only 19 of 309 amino acid residues, corresponding to 94% homology. Analysis of the mglB control region by promoter-probe vectors revealed that two promoters, P1 and P2, constitute the mgl control region (Pmgl). P1 and P2 function in a synergistic way. P1 is the main promoter of the operon; its activity is 20 times the activity of P2. Both promoters are activated by the cyclic adenosine monophosphate catabolite activator protein (cAMP/CAP) complex. While P1 is inactive in the absence of the cAMP/CAP complex, there is residual activity of P2 under these conditions. Studies on the inducibility of the mglBAEC operon using multicopy plasmid promoter-probe vectors were hampered by the titration of the mgl repressor resulting in a partially constitutive expression of the mgl operon. The results indicate that only P1 is responding to induction by D-fucose. A weak promoter, PD, within the P1 region but divergent to it was found. PD is neither stimulated by the cAMP/CAP complex nor by D-fucose. We cloned the gene located downstream to PD and found it to strongly repress the expression of the mgl operon. We termed this gene mglD. The presence of D-fucose abolished the repression caused by the plasmid-encoded mglD gene product.

Characterization of the Salmonella typhimurium mgl operon and its gene products

In Salmonella typhimurium and Escherichia coli the high-affinity galactose transport system, which contains a periplasmic galactose-binding protein as an essential component, is encoded by the mgl genes. The entire mgl region of S. typhimurium is contained on a 6.3-kilobase EcoRI restriction fragment, which has been cloned into plasmid vectors. We determined the extent of the mgl region on this fragment by Tn5 mutagenesis, examination of lacZ fusions to mgl genes, and subcloning smaller restriction fragments. Polyacrylamide gel electrophoresis of protein preparations derived from strains carrying different plasmids was used to identify the mgl gene products. We conclude that the mgl operon consists of four genes that form a single transcription unit: mglB, mglA, mglE, and mglC. The mglB gene codes for galactose-binding protein (33,000 daltons), mglA codes for a membrane-bound protein of 51,000 daltons, and mglC codes for a 29,000-dalton membrane protein. The mglE product was less well characterized. Its existence was inferred from a mglE-lacZ protein fusion located between mglA and mglC. In addition, the coupled transcription-translation in vitro system indicated that mglE codes for a 21,000-dalton protein.

Defective secretion of maltose- and ribose-binding proteins caused by a truncated periplasmic protein in Escherichia coli

The secretion in Escherichia coli of a C-terminally truncated periplasmic enzyme from Salmonella typhimurium, the glpQ-encoded glycerolphosphate phosphodiesterase, was studied. Plasmid pRH100, carrying the truncated glpQ gene, directs the synthesis of a 30,000-molecular-weight (30 K) protein that is processed to a mature 27.5 K protein. (The mature wild-type protein is a 38 K protein.) The truncated protein is not released into the periplasm but remains membrane associated, although it becomes protease sensitive after conversion of cells to spheroplasts. The presence of pRH100 strongly reduces the amount of some other proteins in the periplasm, including the maltose- and ribose-binding proteins. The reduction does not occur at the level of transcription or early translation, as shown by lacZ fusions to the gene coding for the structural gene of the maltose-binding protein. Outer membrane proteins are not affected. A hydroxylamine-induced mutation in the sequence of glpQ corresponding to the mature polypeptide overcomes the inhibitory effect of pRH100. The mutated gene no longer directs the synthesis of the 30/27.5 K protein but directs that of a new 19 K protein which is not membrane bound. We propose that sorting signals in the mature GIpQ protein are necessary for effective translocation to the periplasm and that the C-terminal third of the protein is essential for release into the periplasm.

Cloning of the ugp region containing the structural genes for the pho regulon-dependent sn-glycerol-3-phosphate transport system of Escherichia coli

Using a novel positive selection method for G3P transport activity, lambda phages that carry either all or part of ugp, the genes of the pho regulon-dependent G3P transport system of Escherichia coli were isolated from a library of EcoRI fragments of Escherichia coli established in lambda gt7. By subcloning EcoRI fragments carried by the different phages into the multicopy plasmids pACYC184 and pUR222, it was shown that two chromosomal fragments of 6.0 and 6.6 kb are required for the expression of ugp, whereas all the structural information is located on the 6.6 kb EcoRI fragment. A restriction map of the cloned DNA was established and the extent of ugp genes determined by Tn5 insertions. Using ugp-lacZ fusions, it could be shown that the ugp region consists of at least two different operons that are transcribed in the same direction (counterclockwise) on the E. coli chromosome.

Reconstitution of maltose transport in Escherichia coli: conditions affecting import of maltose-binding protein into the periplasm of calcium-treated cells

The reconstitution of active transport by the Ca2+ -induced import of exogenous binding protein was studied in detail in whole cells of a malE deletion mutant lacking the periplasmic maltose-binding protein. A linear increase in reconstitution efficiency was observed by increasing the Ca2+ - concentration in the reconstitution mixture up to 400 mM. A sharp pH optimum around pH 7.5 was measured for reconstitution. Reconstitution efficiency was highest at 0 degree C and decreased sharply with increasing temperature. The time necessary for optimal reconstitution at 0 degree C and 250 mM Ca2+ was about 1 min. The competence for reconstitution was highest in exponentially growing cultures with cell densities up to 1 X 10(9)/ml and declined when the cells entered the stationary-growth phase. The apparent Km for maltose uptake was the same as that of wild-type cells (1 to 2 microM). Vmax at saturating maltose-binding protein concentration was 125 pmol per min per 7.5 X 10(7) cells (30% of the wild-type activity). The concentration of maltose-binding protein required for half-maximal reconstitution was about 1 mM. The reconstitution procedure appears to be generally applicable. Thus, galactose transport in Escherichia coli could also be reconstituted by its respective binding protein. Maltose transport in E. coli was restored by maltose-binding protein isolated from Salmonella typhimurium. Finally, in S. typhimurium, histidine transport was reconstituted by the addition of shock fluid containing histidine-binding protein to a hisJ deletion mutant lacking histidine-binding protein. The method is fast and general enough to be used as a screening procedure to distinguish between transport mutants in which only the binding protein is affected and those in which additional transport components are affected.