Fine-structure genetic map of the maltose transport operon of Salmonella typhimurium

Isolation, functional characterization and crystallization of Aq_1259, an outer membrane protein with porin features, from Aquifex aeolicus

The "hypothetical protein" Aq_1259 was identified by mass spectrometry and purified from native membranes of Aquifex aeolicus. It is a 49.4kDa protein, highly homologous (>52% identity) to several conserved hypothetical proteins from other bacteria. However, none of these proteins has been characterized using biochemical or electrophysiological techniques. Based on the sequence and circular dichroism spectroscopy, the structure of Aq_1259 is predicted to be a β-barrel with 16 β-strands. The strands with loops and turns are distributed evenly through the entire sequence. The function of Aq_1259 was analyzed after incorporation into a lipid bilayer. Electrophysiological measurements revealed a pore that has a basic stationary conductance of 0.48 ± 0.038nS in a buffer with 0.5M NaH₂PO₄ at pH 6.5 and 0.2 ± 0.015nS in a buffer with 0.5M NaCl at pH 6.5. Superimposed on this is a fluctuating conductance of similar amplitude. Aq_1259 could be crystallized. The crystals diffract to a resolution of 3.4Å and belong to space group I222 with cell dimensions of a=138.3Å, b=144.6Å, c=151.8Å.

The ATP binding cassette (ABC) transport systems of Mycobacterium tuberculosis

We have undertaken the inventory and assembly of the typical subunits of the ABC transporters encoded by the complete genome of Mycobacterium tuberculosis. These subunits, i.e. the nucleotide binding domains (NBDs), the membrane-spanning domains (MSDs) and the substrate binding proteins (SBPs), were identified on the basis of their characteristic stretches of amino acids and/or conserved structure. A total of 45 NBDs present in 38 proteins, of 47 MSDs present in 44 proteins and of 15 SBPs were found to be encoded by M. tuberculosis. Analysis of transcriptional clusters and searches of homology between the identified subunits of the transporters and proteins characterized in other organisms allowed the reconstitution of at least 26 complete (including at least one NBD and one MSD) and 11 incomplete ABC transporters. Sixteen of them were unambiguously classified as importers whereas 21 were presumed to be exporters. By searches of homology with already known transporters from other organisms, potential substrates (peptides, macrolides, carbohydrates, multidrugs, antibiotics, iron, anions) could be attributed to 30 of the ABC transporters identified in M. tuberculosis. The ABC transporters have been further classified in nine different sub-families according to a tree obtained from the clustering of their NBDs. Contrary to Escherichia coli and similarly to Bacillus subtilis, there is an equal representation of extruders and importers. Many exporters were found to be potentially implicated in the transport of drugs, probably contributing to the resistance of M. tuberculosis to many antibiotics. Interestingly, a transporter (absent in E. coli and in B. subtilis) potentially implicated in the export of a factor required for the bacterial attachment to the eukaryotic host cells was also identified. In comparison to E. coli and B. subtilis, there is an under-representation of the importers (with the exception of the phosphate importers) in M. tuberculosis. This may reflect the capacity of this bacterium to synthesize many essential compounds and to grow in the presence of few external nutrients. The genes encoding the ABC transporters occupy about 2.5% of the genome of M. tuberculosis.

Immunogenicity of viral B-cell epitopes inserted into two surface loops of the Escherichia coli K12 LamB protein and expressed in an attenuated aroA strain of Salmonella typhimurium

We previously developed a general procedure which allows the genetic coupling of a chosen foreign linear epitope in different 'permissive' sites of a carrier protein. By using the outer membrane protein LamB of Escherichia coli K12 as a carrier, we were able to express a number of different foreign epitopes at the bacterial surface. In the present work, taking advantage of the recent determination of the crystal structure of LamB, we inserted two model B-cell epitopes i.e.--the C3 epitope from poliovirus (residues 93 to 103 of VP1) and the preS2 epitope from hepatitis B virus, (residues 132 to 145)--at the tip of the most distal and largest surface exposed region of LamB (after residues 386, into loop L9). We also used two previously constructed LamB hybrids, corresponding to the insertion of the C3B or preSB epitope into permissive site 153 (lying in the middle of the fourth surface loop of LamB), to construct two LamB proteins corresponding to the simultaneous insertion of the two different epitopes (with one epitope per site). The LamB hybrids were placed under the control of the anaerobically inducible pnirB promoter and expressed in a LamB-negative derivative of the aroA attenuated strain of S. typhimurium, SL3261. In vitro, the recombinant proteins were expressed at a high level (up to 10% of whole cell proteins) and in vivo the recombinant plasmids were stably maintained. For both epitopes, genetic coupling at site 386 appeared to be more favorable for the induction of anti-epitope antibodies than coupling at site 153. Moreover, the LamB hybrid corresponding to the simultaneous insertion of the preSB epitope at site 153 and of the C3B epitope at site 386 allowed the induction of both anti-poliovirus and anti-hepatitis B antibodies.

Control by H-2 genes of the Th1 response induced against a foreign antigen expressed by attenuated Salmonella typhimurium

Attenuated salmonellae represent an attractive vehicle for the delivery of heterologous protective antigens to the immune system. Here, we have investigated the influence of the genetic background of the host which regulates the growth and elimination of Salmonella cells on the cellular response induced against a foreign antigen delivered by an aroA Salmonella strain. We have tested CD4+ T-cell responses (cell proliferation and cytokine production) in various mouse strains following immunization with Salmonella typhimurium SL3261 expressing a high level of the recombinant Escherichia coli MalE protein. We were able to detect a CD4+ T-cell response against the recombinant MalE protein only in a restricted number of mouse strains, whereas all mice produced good levels of anti-MalE immunoglobulin G antibodies. The Ity gene did not play a major role in these differences in T-cell responses, since both Ity-resistant and -susceptible strains of mice were found to be unresponsive to MalE delivered by recombinant salmonellae. In contrast, when B10 congenic mice were used, a correlation was established between MalE-specific T-cell unresponsiveness and H-2 genes. The discrepancies described in this paper in the ability of various strains of mice to develop an efficient Th1 response against a recombinant antigen displayed by a live Salmonella vaccine underscore the difficulties that can be encountered in the vaccination of human populations by such a strategy.

Quantification of the regulation of glycerol and maltose metabolism by IIAGlc of the phosphoenolpyruvate-dependent glucose phosphotransferase system in Salmonella typhimurium

The amount of IIAGlc, one of the proteins of the phosphoenolpyruvate:glucose phosphotransferase system (PTS), was modulated over a broad range with the help of inducible expression plasmids in Salmonella typhimurium. The in vivo effects of different levels of IIAGlc on glycerol and maltose metabolism were studied. The inhibition of glycerol uptake, by the addition of a PTS sugar, was sigmoidally related to the amount of IIAGlc. For complete inhibition of glycerol uptake, a minimal ratio of about 3.6 mol of IIAGlc to 1 mol of glycerol kinase (tetramer) was required. Varying the level of IIAGlc (from 0 to 1,000% of the wild-type level) did not affect the growth rate on glycerol, the rate of glycerol uptake, or the synthesis of glycerol kinase. In contrast, the growth rate on maltose, the rate of maltose uptake, and the synthesis of the maltose-binding protein increased two- to fivefold with increasing levels of IIAGlc. In the presence of cyclic AMP, the maximal levels were obtained at all IIAGlc concentrations. The synthesis of the MalK protein, the target of IIAGlc, was not affected by varying the levels of IIAGlc. The inhibition of maltose uptake was sigmoidally related to the amount of IIAGlc. For complete inhibition of maltose uptake by a PTS sugar, a ratio of about 18 mol of IIAGlc to 1 mol of MalK protein (taken as a dimer) was required.

Completion of the nucleotide sequence of the 'maltose B' region in Salmonella typhimurium: the high conservation of the malM gene suggests a selected physiological role for its product

We have subcloned and sequenced the genes malF and malM of Salmonella typhimurium, thereby completing the determination of the nucleotide sequence of its 'maltose B' regulon. The malM gene, encoding a periplasmic protein of unknown function in Escherichia coli, is a highly conserved as genes encoding proteins of known function from the same region.

A chimeric nucleotide-binding protein, encoded by a hisP-malK hybrid gene, is functional in maltose transport in Salmonella typhimurium

We have isolated a hybrid gene, composed of the first 455 nucleotides of hisP and nucleotides 275-1107 of malK, the genes coding for the nucleotide-binding components of the high-affinity transport systems for histidine and maltose in Salmonella typhimurium, respectively. The fusion had occurred by recombination within 11 homologous base pairs located between the two DNA fragments. In the chimeric protein peptidic motifs A and B, proposed to be part of the nucleotide-binding fold, originate from HisP and MalK, respectively. Plasmid pES42-39, harbouring the hybrid gene, was shown to complement only a malK mutation but failed to complement a hisP deletion mutation. The chimeric protein was identified by immunoblotting as a protein with an apparent molecular mass of 49kDa. Removal of the C-terminal 77 amino acid residues from the chimeric protein resulted in the loss of function in transport. In contrast, 51 amino acid residues could be removed from the C-terminus of wild-type MalK without any effect. Upon overproduction the chimeric protein, as wild-type MalK, inhibited expression of the malB regulon. However, both truncated proteins, when overproduced, did not exhibit this activity. Based on these results, a tentative model of the functional domains of MalK is presented.

The maltoporin of Salmonella typhimurium: sequence and folding model

The sequence of the lamB gene from Salmonella typhimurium was determined. It encodes the precursor to the LamB protein from S. typhimurium (pre-LamBS.t.; 452 residues) which presents extensive homologies with the pre-LamB protein from Escherichia coli (pre-LamBE.c.; 446 residues). The first third of pre-LamBS.t. is the most conserved, with 4% changes and strict identity between the signal peptides. The last two-third contains five "variable" segments where more than 50% of the residues are changed with respect to LamBE.c.. The three first variable segments are 8 to 14 residues long and contain only substitutions, while the two more distal ones are 24 and 29 residues long and also include insertions and deletions. It is remarkable that the variable segments correspond essentially to regions predicted to be extramembranous loops on our 2D folding model for LamBE.c.; they alternate with conserved predicted transmembranous segments. Four of the variable regions were predicted to be cell-surface-exposed loops on the basis of genetic and immunological data, while one of them (region II) was predicted to be periplasmic on the sole basis of folding rules. The LamB protein from S. typhimurium can substitute for the LamB protein from E. coli for maltodextrins binding and transport, but not for infection by any of the known E. coli phages using LamBE.c. for adsorption. A tetrapeptide, RGDS, assumed to be responsible for mammalian cell aggregation by LamBE.c. is conserved in LamBS.t., suggesting that it could have a functional role. The conservation of the binding and transport activity can be accounted for by the conservation of the regions known to be directly involved, namely the first third of the protein and a region corresponding to 352 to 374 of LamBS.t.. The phage resistance can be attributed to the variability of the four cell-surface-exposed loops previously identified as essential for phage adsorption. These results, together with those obtained with polyclonal and monoclonal antibodies directed against known LamB regions, strongly support the folding model presented for LamBE.c. and the idea that it can essentially be extended to LamBS.t., except perhaps for a region between residues 155 and 245. We propose that the existence of variable regions is due essentially, and perhaps only, to the local lack of structural constraints in the protein. The intergenic region between lamB and the following gene, malM, comprises conserved segments, including one palindromic unit.

The sequence of the malG gene from Salmonella typhimurium and its functional implications

The nucleotide sequence of the malG gene which is essential for maltose transport was determined in Salmonella typhimurium and compared to homologous genes from Escherichia coli and Enterobacter aerogenes. malG genes from S. typhimurium and E. aerogenes were expressed and their products were active in E. coli. The primary structure of the three MalG proteins was highly conserved. Changes were mainly clustered in a relatively large hydrophilic region of the protein (residues 40 to 75). In contrast, other hydrophilic segments were more conserved, and most remaining changes occurred in the hydrophobic putative transmembrane segments. This suggests that hydrophilic loops in this inner membrane protein may be functionally constrained. These results prove new insights into the functional sites in MalG.