Gene fusions to the ptsM/pel locus of Escherichia coli

We have constructed gene fusions between ptsM/pel and lacZ. These fusions affect both phenotypes assigned to the ptsM/pel locus (at 40 min), namely, no growth on mannose or glucosamine and inhibition of the penetration of bacteriophage lambda DNA, as well as that of other lambdoid phages such as Hy-2. Since the lacZ gene fusions are insertion mutations that abolish target gene function by disrupting the linear contiguity of the gene, it would appear that ptsM and pel are either the same gene or two genes within the same operon. Several size classes of these ptsM/pel-lacZ fusions have been isolated and the corresponding hybrid proteins are associated with the cytoplasmic membrane of Escherichia coli. This is consistent with the proposal that ptsM/pel codes for Enzyme II of the phosphotransferase transport system (PTS) specific for mannose, glucosamine, fructose and glucose. However, we have also identified Tn10 insertion mutations that confer a Man- phenotype but have no effect on the Pel phenotype. Complementation analysis indicates that the Tn10 insertions and the lacZ gene fusions are in different genes. Both of these genes are involved in mannose uptake. This suggests that the locus at 40 min can be subdivided into two genes whose products are required for mannose uptake and that only one of these is involved in the penetration of lambda DNA.

Isolation and characterization of delta ompB strains of Escherichia coli by a general method based on gene fusions

We isolated a series of delta ompR delta envZ mutants by inducing a strain carrying a lambda prophage in the closely linked gene malP and screening the bacterial survivors for loss of the major outer membrane porins OmpF and OmpC. Characterization of these deletion strains showed that both OmpR and EnvZ were necessary for transcription of ompF and ompC and that neither gene was essential for cell viability. Moreover, the deletion strains did not exhibit the pleiotropic membrane protein deficiency observed with certain envZ mutants. The method described should allow the simple isolation of deletions in any region of the chromosome.

Intragenic regions required for LamB export

Escherichia coli strains containing a series of lamB-lacZ fusions have been isolated and characterized. Each of these fusions specifies a hybrid protein with LamB sequences at the NH2 terminus and a large functional COOH-terminal fragment of beta-galactosidase. The amount of LamB present in the various hybrid proteins ranges from as few as 4 amino acids to a complete signal sequence (25 amino acids) plus 49 amino acids of the mature protein. With respect to hybrid protein export these fusions fall into three classes. Hybrid proteins with an incomplete LamB signal sequence or those that have a complete signal sequence plus 27 or fewer amino acids of the mature LamB protein are not exported and remain in the cytoplasm. In contrast, fusion strains attempt to export hybrid proteins that contain a complete signal sequence plus 39 or 43 amino acids of mature LamB. However, these proteins are not localized to the outer membrane. Finally, a hybrid protein that is slightly larger, containing 49 amino acids of mature LamB, is found in the outer membrane in appreciable amounts. These fusions, together with previously described lamB-lacZ fusions, have enabled us to define more precisely the minimal amount of lamB required to initiate the process of protein export. Moreover, they genetically locate a signal that appears to guide LamB to the outer membrane. This signal is within a region of amino acid homology shared by other major outer membrane proteins [ Nikaido , H. & Wu, H. C. P. (1984) Proc. Natl. Acad. Sci. USA 81, 1048-1052].

Lambda placMu: a transposable derivative of bacteriophage lambda for creating lacZ protein fusions in a single step

We isolated a plaque-forming derivative of phage lambda, lambda placMu1 , that contains sequences from bacteriophage Mu enabling it to integrate into the Escherichia coli chromosome by means of the Mu transposition system. The Mu DNA carried by this phage includes both attachment sites as well as the cI, ner (cII), and A genes. Lambda placMu1 also contains the lacZ gene, deleted for its transcription and translation initiation signals, and the lacY gene of E. coli, positioned next to the terminal 117 base pairs from the S end of Mu. Because this terminal Mu sequence is an open reading frame fused in frame to lacZ, the phage can create lacZ protein fusions in a single step when it integrates into a target gene in the proper orientation and reading frame. To demonstrate the use of this phage, we isolated lacZ fusions to the malB locus. These showed the phenotypes and regulation expected for malB fusions and could be used to isolate specialized transducing phages carrying the entire gene fusion as well as an adjacent gene (malE). They were found to be genetically stable and rarely (less than 10(-7] gave rise to secondary Lac+ insertions. We also isolated insertions into high-copy-number plasmids. The physical structure of these phage-plasmid hybrids was that expected from a Mu-dependent insertion event, with the lambda placMu prophage flanked by the Mu attachment sites. Lac+ insertions into a cloned recA gene were found at numerous positions and produced hybrid proteins whose sizes were correlated with the position of the fusions in recA.

Mutation prlF1 relieves the lethality associated with export of beta-galactosidase hybrid proteins in Escherichia coli

The 42-1 lamB-lacZ gene fusion confers a conditionally lethal, export-dependent phenotype known as maltose sensitivity. A maltose-resistant mutant showing decreased beta-galactosidase activity of the hybrid protein, designated prlF1 (protein localization), was unlinked to the lamB-lacZ fusion. This mutation mapped at 70 min on the Escherichia coli linkage map and conferred maltose resistance, a 30-fold reduction in beta-galactosidase activity, and a 30% decrease in cellular growth rate at 30 degrees C that was independent of the presence of a gene fusion. prlF1 also decreased the beta-galactosidase activity and relieved the maltose sensitivity conferred by fusions of lacZ to the gene specifying the periplasmic maltose-binding protein, malE. The decrease in beta-galactosidase activity, however, was specific for exported hybrid proteins. When export of the hybrid protein was blocked by a signal sequence mutation, prlF1 decreased the beta-galactosidase activity only 2.5-fold. Similarly, prlF1 did not affect the beta-galactosidase activity of fusions of lacZ to a gene specifying a nonexported protein, malK.

D-ribose metabolism in Escherichia coli K-12: genetics, regulation, and transport

We have isolated mutants defective in high-affinity D-ribose transport. The mutations map in rbsT or rbsB , the structural gene for ribose binding protein. rbsT consists of at least one gene coding for a protein required for high-affinity transport. The high-affinity transport-defective mutants were able to utilize D-ribose, indicating that at least a second, low-affinity transport system for D-ribose is present in Escherichia coli K-12. rbsT and rbsB are located at min 84 on the E. coli genetic map and, together with rbsK , the gene coding for ribokinase , constitute an rbs operon. The order of genes is rbsP /O rbsT rbsB rbsK . The rbs operon is subject to negative control by the product of the rbsR gene. rbsR is located distal to the rbs operon and appears to form a separate transcriptional unit.

lacZ fusions to genes that specify exported proteins: a general technique

We have devised a general, one-step technique for isolation of strains in which the gene coding for an exported protein is fused to the gene for beta-galactosidase (lacZ). These fusions specify a hybrid protein comprised of an NH2-terminal portion of the exported protein and a large functional COOH-terminal portion of beta-galactosidase. The fusions are constructed with a derivative of the MudII (lac, Ap) phage. To overcome the lethality that is often associated with the expression of such a hybrid gene, we have recombined an early lacZ nonsense mutation onto this phage. With the use of strains that carry a temperature-sensitive nonsense suppressor, expression of the full-length hybrid protein can be controlled by varying the growth temperature. We demonstrated the utility of this technique by isolating a series of fusions to a gene, ompA, coding for a major outer membrane protein. As expected, strains containing these fusions are not viable under conditions that permit synthesis of a functional nonsense suppressor. Accordingly, this method should also be useful for direct selection of export-defective mutants.

Isolation and characterization of chain-terminating nonsense mutations in a porin regulator gene, envZ

We isolated four independent amber mutations in gene envZ, whose product is involved in the regulation of porin protein genes ompF and ompC. The envZ amber strains exhibit an OmpF-/+ OmpC- porin phenotype and express other envelope protein genes at wild-type levels. This phenotype is clearly different from that of the previously isolated class of envZ mutants that exhibit an OmpF- OmpC+ phenotype and a pleiotropic decrease in the expression of several exported protein genes, including lamB and phoA. The addition of the local anesthetic procaine to wild-type strains also causes a pleiotropic decrease in the expression of genes ompF, lamB, and phoA. However, procaine has no effect on the synthesis of LamB or PhoA protein in the envZ amber strains. Thus, although EnvZ protein is required for the full expression of ompF and ompC, it apparently is not normally involved in the expression of other envelope protein genes. One interpretation of these results is that the EnvZ protein can be altered either by mutation or by procaine to a form that interferes with the expression of several envelope protein genes other than ompF and ompC. Finally, complementation analysis with ompR insertion mutations supports the physical data of Mizuno et al. (J. Biol. Chem 257:13692-13698, 1982) that suggest that envZ is cotranscribed with ompR from a single promoter in the order ompR envZ.

Importance of secondary structure in the signal sequence for protein secretion

Mutant Escherichia coli strains in which export of the LamB protein (coded for by the lamB gene) to the outer membrane of the cell is prevented have been described previously. One of these mutant strains contains a small (12-base pair) deletion mutation within the region of the lamB gene that codes for the NH2-terminal signal sequence. In this mutant strain, export but not synthesis of the LamB protein is blocked. We have isolated pseudorevertants that restore export of functional LamB protein to the outer membrane. DNA sequence analysis showed that two of the revertants contain a point mutation in addition to the original deletion. These point mutations lead to amino acid substitutions within the signal sequence. Our results indicate that these secondary mutations efficiently suppress the export defect caused by the deletion mutation. Analysis of the secondary structure of the wild-type, mutant, and pseudorevertant LamB signal sequences suggests that the secondary mutations restore export by allowing the formation of a stable alpha-helical conformation in the central, hydrophobic region of the signal sequence.

Open reading frame expression vectors: a general method for antigen production in Escherichia coli using protein fusions to beta-galactosidase

We have developed an Escherichia coli plasmid vector for the identification and expression of foreign DNA segments that are open reading frames (ORFs). The 5' end of ompF, an E. coli gene encoding an abundant outer membrane protein, is used to provide a strong, regulated promoter, translation initiation site, and signal sequence for export from the cytoplasm. This sequence is coupled to the lacZ gene of E. coli so that expression of beta-galactosidase requires ompF transcription and translation signals. However, this hybrid gene is LacZ- because lacZ is out of frame with respect to ompF. Restriction enzyme recognition sites are located between ompF and lacZ to allow convenient insertion of DNA fragments. If an insert is an ORF of the correct length, ompF and lacZ become realigned in frame, resulting in a LacZ+ gene that produces a tribrid protein with the translation product of the insert sandwiched between OmpF and beta-galactosidase. The LacZ+ phenotype thus identifies clones containing an expressed ORF. To demonstrate the vector's utility we inserted a fragment from the herpes virus thymidine kinase gene and used the resulting tribrid protein to raise antibodies that precipitate thymidine kinase from herpes virus-infected cells. We also inserted a fragment from the E. coli lexA gene to produce a tribrid protein that is precipitated by antiserum raised with LexA protein. Thus, tribrid fusion proteins can be used to produce or detect antibodies and also to identify the product of a cloned gene.

Isolation and characterization of mutations altering expression of the major outer membrane porin proteins using the local anaesthetic procaine

Mutations at several different chromosomal locations affect expression of the major outer membrane porin proteins (OmpF and OmpC) of Escherichia coli K12. Those that map at 21 and 47 minutes define the structural genes for OmpF and OmpC, respectively. A third locus, ompB, is defined by mutations that map at 74 minutes. The ompB locus contains two genes whose products regulate the relative amounts of ompF and ompC expression. One of these genes, ompR, encodes a positive regulatory protein that interacts at the ompF and ompC promoters. Mutations in ompR exhibit an OmpF- OmpC- or an OmpF+ OmpC- phenotype. The product of the second gene, envZ, affects regulation of the porin proteins in an unknown manner. Previously isolated mutations in envZ exhibit an OmpF- OmpC+ phenotype and also have pleiotropic effects on other exported proteins. In the presence of local anaesthetics such as procaine, wild-type strains exhibit properties similar to these envZ mutants, i.e. OmpF- OmpC+. Using ompF-lac fusion strains, we have exploited this procaine effect to isolate two new classes of envZ mutations. One of these classes exhibits an OmpF+ OmpC- phenotype. The other allows expression of both OmpF and OmpC but alters the relative amounts found under various growth conditions. Like previously isolated envZ mutations, these also affect regulation of other exported proteins, such as lambda receptor. These results permit a more detailed analysis of the omp regulon and they may shed light on one of the mechanisms by which local anaesthetics exert their effect.

Information within the mature LamB protein necessary for localization to the outer membrane of E coli K12

It has been proposed that the efficient localization of the outer membrane protein LamB requires a functional signal sequence and at least two additional regions contained within the mature protein. We define these regions more precisely by deletion analysis, and we describe methods for cloning deleterious lacZ fusions onto high-copy-number plasmids and generating in-frame deletions. Analysis of the effects of a series of internal lamB deletions on the export of a LamB-LacZ hybrid protein and of the LamB protein itself indicates that necessary informational signal(s) required for localization lie at the amino-terminal end of the protein. In addition, our analysis indicates that there is a region of information close to or within the fusion joint of the largest lamB-lacZ fusion that increases the efficiency of the export process. A unique deletion that removes a protein segment from amino acid 70 to 200 appears to prevent proteolytic removal of the signal sequence. Nevertheless, the mutant protein is exported to the outer membrane.

Genetic analysis of protein export in Escherichia coli

Genetic studies on the secretion process in gram-negative bacteria have made considerable progress. Within the near future, such studies should lead to a detailed understanding of the important features of signal sequences and how they function. The cloning of the structural gene for an enzyme that cleaves signal sequences from precursors of secreted proteins will allow the genetic characterization of this locus and its function. Finally, the isolation and characterization of mutants that affect components of the cell's secretory apparatus are also under way. These mutants permit the detection of genes and their products that are involved in secretion. A combination of the genetic approaches and in vitro studies should lead to a picture of the details of passage of proteins through a membrane.

Molecular components of the signal sequence that function in the initiation of protein export

We are studying the mechanism by which the LamB protein is exported to the outer membrane of Escherichia coli. Using two selection procedures based on gene fusions, we have identified a number of mutations that cause alterations in the LamB signal sequence. Characterization of the mutant strains revealed that although many such mutations block LamB export to greater than 95%, others have essentially no effect. These results allow an analysis of the functions performed by the various molecular components of the signal sequence. Our results suggest that a critical subset of four amino acids is contained within the central hydrophobic core of the LamB signal sequence. If this core can assume an alpha-helical conformation, these four amino acids comprise a recognition site that interacts with a component of the cellular export machinery. Since mechanisms of protein localization appear to have been conserved during evolution, the principles established by these results should be applicable to similar studies in eukaryotic cells.

A previously unidentified gene in the spc operon of Escherichia coli K12 specifies a component of the protein export machinery

The gene prlA codes for a factor that appears to function in the export of proteins in Escherichia coli. This conclusion is based on the finding that mutations altering the prlA gene product restore export of envelope proteins with defective signal sequences. Previous results showed that the prlA gene lies in an operon (spc) known to code for ten different ribosomal proteins. Our studies show that the prlA gene lies promoter-distal to the last known ribosomal protein gene in this operon. Evidence from gene fusions constructed in vitro suggests that prlA codes for a protein containing at least 300 amino acids. Thus a heretofore unidentified protein specified by a gene within the spc operon appears to be a component of the cellular protein export machinery.

Identification of OmpR: a positive regulatory protein controlling expression of the major outer membrane matrix porin proteins of Escherichia coli K-12

We report here on the cloning of a gene located within the ompB locus of E. coli K-12. This gene, designated ompR, resides on a 10.9-kilobase EcoRI fragment that we cloned, using a lambda vector and in vitro packaging techniques. By subcloning portions of this fragment into the high-copy-number plasmid pBR322, we have isolated the ompR gene on a 1.4-kilobase EcoRI-AvaI fragment. This fragment has been characterized physically and will facilitate a more detailed study of the role and mechanism of porin regulation by the ompB locus.

Identification of the malK gene product. A peripheral membrane component of the Escherichia coli maltose transport system

The malK gene product of Escherichia coli has been identified through the use of a previously described technique that employs gene fusions (Shuman, H. A., Silhavy, T. J., and Beckwith, J. R. (1980) J. Biol. Chem. 255, 168-174). This protein, along with the four other products of the malB locus, comprise the complete maltose transport system. The malK protein has a molecular weight of approximately 40,000 and is located in the cell envelope. In mutant strains which lack another component of the transport system, the malG protein, the malK protein is located in the cytoplasm. This alteration in location suggests that the malK protein is associated with the inner surface of the cytoplasmic membrane via an interaction with the malG protein.

Suppressor mutations that restore export of a protein with a defective signal sequence

A selection procedure is described that should allow the genetic identification of cellular components involved in the process of protein localization in Escherichia coli. This procedure makes use of mutations that alter the signal sequence of the lambda receptor protein (product of the lamB gene), and prevent export of this protein to its normal outer membrane location. Several suppressor mutations have been identified that restore export of the mutant lambda receptor protein. Mapping experiments show that the suppressor phenotype is the result of mutations in any of at least three different chromosomal loci. One class of suppressor mutations, the class containing the largest number of independent isolates, maps in the major ribosomal gene cluster, suggesting that the suppressor phenotype is the consequence of an altered ribosomal protein. This class of suppressors phenotypically suppresses all known export-defective mutations, internal to the signal sequence region of the lamB gene. These results suggest that ribosomes play an important role in the export of lambda receptor to the outer membrane.

A mechanism of protein localization: the signal hypothesis and bacteria

We are studying the molecular mechanism of cellular protein localization. The availability of genetic techniques, such as gene fusion in Escherichia coli, has made this problem particularly amenable to study in this prokaryote. We have constructed a variety of strains in which the gene coding for an outer membrane protein is fused to the gene coding for a normally cytoplasmic enzyme, beta-galactosidase. The hybrid proteins produced by such strains retain beta-galactosidase activity; this activity serves as a simple biochemical tag for studying the localization of the outer membrane protein. In addition, we have exploited phenotypes exhibited by certain fusion strains to isolate mutants that are altered in the process of protein export. Genetic and biochemical analyses of such mutants have provided evidence that the molecular mechanism of cellular protein localization is strinkingly similar in both bacteria and animal cells.

A signal sequence is not sufficient to lead beta-galactosidase out of the cytoplasm

Escherichia coli strains have been constructed in which lacZ, the gene for the cytoplasmic enzyme beta-galactosidase, is fused to lamB, the gene for an outer membrane protein. One such strain produces a beta-galactosidase which remains cytoplasmic even though it possesses the complete signal sequence of the lamB protein precursor at the amino-terminal end.

Sequence analysis of mutations that prevent export of lambda receptor, an Escherichia coli outer membrane protein

The amino-terminal signal sequence is required for initiation of transmembrane protein transfer of the Escherichia coli lambda receptor protein. Mutations leading to insertion of charged amino acids into or deletion of amino acids from the hydrophobic segment of this sequence prevent export of this outer membrane protein.

Genetic studies on mechanisms of protein localization in Escherichia coli K-12

In the last few years, several laboratories have demonstrated that many proteins (both from eukaryotic and prokaryotic organisms) that are destined to be localized in noncytoplasmic locations initially are synthesized as a precursor with a 15-30 amino acid extension at the NH2-terminal end of the molecule. This extra peptide has been termed the signal sequence, and it has been proposed that this signal plays a role in the localization of the extracytoplasmic protein. We are studying the process by which proteins are exported to the envelope region of Escherichia coli. Our work deals primarily with the outer membrane proteins, lambda receptor, the product of the lamB gene, and the major outer membrane (porin) proteins 1a and 1b, products of the ompF and ompC genes. Using techniques of gene fusion, we have demonstrated that information specifying the cellular location of the lambda receptor is contained within the lamB gene. Furthermore, we have shown that this information is capable of directing even a normally cytoplasmic protein, beta-galactosidase, to the outer membrane. Some of this information is contained within the signal sequence. Mutations that alter this sequence prevent export of the lambda receptor protein. Again using techniques of gene fusion, we have shown that the signal sequence alone is not sufficient to cause export of beta-galactosidase from the cytoplasm. Other information within the lamB gene is required. Selection procedures have been developed to isolate mutations that exhibit a general alteration in the export process. Genetic analysis of these mutations has provided evidence for the involvement of the ribosome in the process of protein localization. The structural genes for the porin proteins, 1a and 1b, are regulated at the transcriptional level by the ompB locus. This has permitted us to extend our studies on outer membrane protein localization to protein 1. With this genetic system, it should be possible to determine if E coli employs more than a single mechanism for the export of proteins to the outer membrane.

Transcriptional regulation of Escherichia coli K-12 major outer membrane protein 1b

Eleven independent insertion mutations were isolated that prevented expression of major outer membrane protein 1b. Seven of the mutations were Mucts insertions located at ombP. These ompB::Mucts strains fell into two phenotypic classes with regard to expression of proteins 1a and 1b. The remaining four mutants were comprised of one Tn5 and three Mucts insertions mapping at par. The Mucts insertions at par were used to construct fusions of the lac operon to the par promoter. Expression of beta-galactosidase in these fusion strains reflected known regulatory properties of protein 1b. When an ompB allele was introduced into the par-lac fusion strains, beta-galactosidase activity was reduced 14- to 31-fold. Transcriptional regulation of the par gene and the existence of two functions at ompB are discussed. The results suggest that par is the structural gene for protein 1b and that an ompB gene product is a diffusible, positive regulatory element controlling expression of par.

Structure of the malB region in Escherichia coli K12. II. Genetic map of the malE,F,G operon

Starting with a strain containing a malK-lacZ fusion, a series of lambda plaque-forming phages which carry varying amounts of the malE,F operon have been isolated. We have used these phages to construct a deletion map of the malE,F operon. The construction of this deletion map has led to the identification of a new gene, malG. The malG gene is located distal to malF. The malG gene product is a protein required for the active transport of maltose and maltodextrins.

Sites within gene lacZ of Escherichia coli for formation of active hybrid beta-galactosidase molecules

We describe the genetic analysis of 21 Escherichia coli strains in which the amino-terminal sequence of beta-galactosidase has been removed and replaced by an amino-terminal sequence from one or another of the proteins involved in maltose transport. Genetic mapping of the lacZ end of these fused genes indicates that only those fusions in which fewer than 41 amino acids are removed from the amino-terminal sequence of beta-galactosidase result in enzymatically active molecules. Within the region between amino acid 17 and amino acid 41 there are at least four or five sites where enzymatically active hybrid proteins can be formed.

Use of gene fusion to study secretion of maltose-binding protein into Escherichia coli periplasm

We have employed the technique of gene fusion to fuse the LacZ gene encoding the cytoplasmic enzyme beta-galactosidase with the malE gene encoding the periplasmic maltose binding protein (MBP). Strains were obtained which synthesize malE-lacZ hybrid proteins of various sizes. These proteins have, at their amino terminus, a portion of the MBP and at their carboxyl terminus, enzymatically active beta-galactosidase. When the hybrid protein includes only a small, amino-terminal portion of the MBP, the hybrid protein residues in the cytoplasm. When the hybrid protein contains enough of the MBP to include an intact MBP signal sequence, a significant portion of the hybrid protein is found in the cytoplasmic membrane, suggesting that secretion of the hybrid protein has been initiated. However, in no case is the hybrid protein secreted into the periplasm, even when the hybrid protein includes almost the entire MBP. In the latter case, the synthesis and attempted export of the hybrid protein interferes with the export of at least certain normal envelope proteins, which accumulate in the cell in their precursor forms, and the cell dies. These results suggest that a number of envelope proteins may be exported at a common site, and that there are only a limited number of such sites. Also, these results indicate that it is not sufficient to simply attach an amino-terminal signal sequence to a polypeptide to assure its export.

Resolution of glpA and glpT loci into separate operons in Escherichia coli K-12 strains

The independent insertion of bacteriophage Mu into the gene coding for anaerobic sn-glycerol 3-phosphate dehydrogenase (glpA) or into the genes coding for sn-glycerol 3-phosphate transport (glpT) suggested that these two closely linked loci are in separate operons.

Mutations altering the cellular localization of the phage lambda receptor, an Escherichia coli outer membrane protein

Two mutant strains of Escherichia coli have been isolated in which the cellular location of an outer membrane protein, the phage lambda receptor (the lamB gene product), is altered. These mutations were initially selected in a strain containing a lamB-lacZ fusion. In the parent strain the protein coded for by the hybrid gene is located, at least in part, in the outer membrane. In the mutants it is located in the cytoplasm. The mutations responsible for the alteration of cellular location lie very early in the lamB gene, in a region corresponding to the NH2-terminus of the lambda receptor protein. One of these mutations is a small deletion internal to the lamB gene. When this mutation is present in an otherwise wild-type lamB gene, the protein produced is of lower molecular weight than normal receptor. The other mutation behaves as a point mutation; when it is present in an otherwise normal lamB gene, reversion can be demonstrated. The molecular weight of this mutant protein, which is located in the cytoplasm, is larger than that of the wild-type gene product by approximately 2000. It is suggested that these two mutations are in the portion of the lamB gene coding for a signal sequence and thereby block export of the protein.

A second transport system for sn-glycerol-3-phosphate in Escherichia coli

Strains containing phage Mucts inserted into glpT were isolated as fosfomycin-resistant clones. These mutants did not transport sn-glycerol-3-phosphate, and they lacked GLPT, a protein previously shown to be a product of the glpT operon. By plating these mutants on sn-glycerol-3-phosphate at 43 degrees C, we isolated revertants that regained the capacity to grow on G3P. Most of these revertants did not map in glpT and did not regain GLPT. These revertants exhibited a highly efficient uptake system for sn-glycerol-3-phosphate within an apparent Km of 5 micron. In addition, three new proteins (GP 1, 2, and 3) appeared in the periplasm of these revertants. None of these proteins were antigentically related to GLPT. However, like GLPT, GP1 exhibits abnormal behavior on sodium dodecyl sulfate-polyacrylamide gels. GP 2 is an efficient binding protein. The new uptake system showed different characteristics than the system that is coded for by the glpT operon. It was inhibited neither by phosphate nor fosfomycin. So far, none of the systems that transport organic acids in Escherichia coli could be implicated in the new sn-glycerol-3-phosphate uptake activity. The mutation ugp+, which was responsible for the appearance of the new transport system and the appearance of GP 1, 2, and 3 in the periplasm was cotransducible with araD by phage P1 transduction and was recessive in merodiploids.

Use of gene fusions to study outer membrane protein localization in Escherichia coli

Escherichia coli strains have been isolated that produce hybrid proteins comprised of an NH2-terminal sequence from the lamB gene product (an outer membrane protein) and a major portion of the COOH-terminal sequence of beta-galactosidase (beta-D-galactoside galactohydrolase, EC 3.2.1.23; a cytoplasmic protein). These proteins exhibit beta-galactosidase activity. One such strain, pop 3105, produces a hybrid protein containing very little of the lamB gene protein; the protein is found in the cytoplasm. The protein found in a second strain, pop 3186, contains much more of the lamB gene protein; a substantial fraction of the beta-galactosidase activity is found in the outer membrane, probably facing outward. These results indicate that information necessary to direct the lamB gene product to its outer membrane location is located within the lamB gene itself. The properties of such fusion strains open up the prospect of a precise genetic analysis of the genetic components involved in protein transport.

Conversion of beta-galactosidase to a membrane-bound state by gene fusion

We have isolated a series of strains in which the lacZ gene has been fused to one of the maltose operons, such that the synthesis of beta-galactosidase (beta-D-galactoside galactohydrolase; EC 3.2.1.23) is inducible by maltose. The most frequent event that generates such fusions results in strains in which an intact lacZ gene has become a part of the malE,F operon. By using a special selection procedure, we have detected much rarer fusion events resulting in an altered beta-galactosidase molecule. In these strains, we presume that there is a hybrid protein molecule produced, comprised of an NH2-terminal amino acid sequence from a maltose transport protein (malF) and a COOH-terminal amino acid sequence from beta-galactosidase. The hybrid protein, which still retains some beta-galactosidase activity, is found in the cytoplasmic membrane. These results provide information on the component of the malF gene essential for incorporation of its product into the membrane.

Maltose transport in Escherichia coli K12. A comparison of transport kinetics in wild-type and lambda-resistant mutants as measured by fluorescence quenching

The kinetic parameters for the maltose transport system in Escherichia coli K12 were determined with maltose and maltotriose as substrates. The system exhibits an apparent Km of 1 muM for maltose and 2 muM for maltotriose. The V of entry was determined as 2.0 and 1.1 nmol substrate/min per 10(8) cells. Mutations in lamB, the structural gene for the receptor protein of phage lambda, increased the Km for maltose transport by a factor of 100-500 without influencing the maximal rate of transport. Maltotriose is no longer transported in these lamB mutants. The maltose-binding protein, an essential component of the maltose transport system, was found to exhibit substrate-dependent fluorescence quenching. This phenomenon was used to determine dissociation constants and to estimate the rate of ligand dissociation. A Kd of 1 muM for maltose and of 0.16 muM for maltotroise was found. From the comparison of the kinetic parameters of transport of maltose and maltotriose in wild-type and lambda-resistant mutants with the binding constants for both sugars to purified maltose-binding protein, we conclude that the lambda receptor facilitates the diffusion of maltose and maltodextrins through the outer membrane.

Periplasmic protein related to the sn-glycerol-3-phosphate transport system of Escherichia coli

Two-dimensional gel electrophoresis of shock fluids of Escherichia coli K-12 revealed the presence of a periplasmic protein related to sn-glycerol-3-phosphate transport (GLPT) that is under the regulation of glpR, the regulatory gene of the glp regulon. Mutants selected for their resistance to phosphonomycin and found to be defective in sn-glycerol-3-phosphate transport either did not produce GLPT or produced it in reduced amounts. Other mutations exhibited no apparent effect of GLPT. Transductions of glpT+ nalA phage P1 into these mutants and selection for growth on sn-glycerol-3-phosphate revealed a 50% cotransduction frequency to nalA. Reversion of mutants taht did not produce GLPT to growth on sn-glycerol-3-phosphate resulted in strains that produce GLPT. This suggests a close relationship of GLPT to the glpT gene and to sn-glycerol-3-phosphate transport. Attempts to demonstrate binding activity of GLPT in crude shock fluid towards sn-glycerol-3-phosphate have failed so far. However, all shock fluids, independent of their GLPT content, exhibited an enzymatic activity that hydrolyzes under the conditions of the binding assay, 30 to 60% of the sn-glycerol-3-phosphate to glycerol and inorganic orthophosphate.

On the significance of the retention of ligand by protein

When a solution of binding protein and its ligand is dialyzed against a large volume of ligand-free medium the rate of exit of the ligand from the protein-containing compartment can be extremely slow, much slower than the rate observed in the absence of protein. This is what we call retention of ligand by protein. A simple calculation demonstrates that when the protein concentration is in large excess over the total ligand concentration, the exit of ligand follows quasi-first-order kinetics, the half-life being proportional to (1 + (P)/Kd), where (P) is the concentration of binding sites, and Kd the dissociation constant characteristic of the equilibrium between the ligand and the protein. Experimental verification of this relation is provided in the case of the periplasmic maltose-binding protein of Escherichia coli; The implications of the retention effect in biochemical techniques are discussed, as well as its possible significance in biological phenomena, such as bacterial chemotaxis and transport, mechanism of hormone action, or transmission of the nerve impulse.

The "hidden ligand" of the galactose-binding protein

Following tryptophan fluorescence of the galactose-binding during dissociation of the ligand it has been found that glucose dissociates with a half life of less than 5 s. Similarly, fast dissociation was also observed by following release of radioactively labelled glucose from Sepharose-coupled galactose-binding protein upon dilution. Accordingly, a previous claim that the galactose-binding protein contains glucose as a non-dissociable "hidden ligand" [G. Richarme and A. Kepes (1974) Eur. J. Biochem. 45, 127-133] has to be reinterpreted