The ferrichrome receptor protein (tonA) of Escherichia coli is synthesised as a precursor in vitro

Prediction of signal sequence-dependent protein translocation in bacteria: assessment of the Escherichia coli minicell system

The use of phenethyl alcohol (PEA) as a probe for signal sequence-dependent protein translocation in minicells was examined. Processing of beta-lactamases and tonA was inhibited by PEA at concentrations which did not affect production of the alpha and gamma forms of penicillin binding protein (PBP) lb. The PBPlbs are believed to lack leader sequences whereas the other proteins contain them. Processing of a beta-lactamase which shares the murein-lipoprotein export pathway was relatively resistant to PEA, consistent with previous findings in whole bacteria. The results reported here suggest that PEA is a suitable probe for leader sequences in the minicell system. By using PEA we predict that PBP4 does not require a leader sequence for membrane insertion.

Role of SecA and SecY in protein export as revealed by studies of TonA assembly into the outer membrane of Escherichia coli

The growth of secAts or secYts mutants at the restrictive temperature has been shown to inhibit the export of many outer membrane proteins. We report here that in two secAts strains the rate of incorporation of newly synthesized protein into both inner and outer membrane fractions decreased by about 70% at the restrictive temperature. The export of the outer membrane protein TonA was used as a model system in which to study the effects of SecA or SecY inactivation. pre-TonA that accumulated at the restrictive temperature was found to co-sediment with the outer membrane fraction. However, the precursor was sensitive to protease and did not float up a sucrose gradient with the membrane fractions. It was therefore concluded that pre-TonA was not integrated into the outer membrane fraction but probably accumulated in the cytoplasm. Studies on the rate of processing of pre-TonA, pulse-labelled at the restrictive temperature then chased at the permissive temperature, revealed differences between secA and secY mutants. In the secAts mutant the great majority of cytoplasmic pre-TonA was not apparently processed to the mature form, whereas in the secYts mutant significant amounts of precursors were rapidly chased into mature TonA, which appeared in the outer membrane. These results suggest that SecA and SecY may act sequentially in the export of proteins to the outer membrane. In particular these data indicate that SecA is required to maintain pre-TonA in a translocationally competent form prior to interaction with the SecY export site.

Intermediates in the assembly of the TonA polypeptide into the outer membrane of Escherichia coli K12

The tonA gene of Escherichia coli K12 was cloned into a multicopy plasmid, leading to substantial overproduction of the corresponding 78,000 Mr polypeptide in the outer membrane. The approximate size of the tonA gene and its direction of transcription were established by Tn1000 mutagenesis. A family of tonA deletions was constructed in vitro by Bal31 exonuclease digestion, followed by splicing of an "oligo stop" sequence to each 3' terminus in order to ensure prompt termination of translation of the truncated polypeptides in vivo. All these polypeptides proved to be extremely unstable in exponentially growing cultures but were relatively stable in maxicells. Under these conditions the truncated polypeptides, unlike wild-type TonA, fractionated with the Sarkosyl-soluble fraction of the cell envelope, indicating that these proteins are blocked in assembly as inner membrane (translocation) intermediates or as outer membrane (maturation) intermediates unable to form Sarkosyl-resistant complexes. We have also examined the kinetics of assembly of wild-type TonA into the outer membrane and the results indicate that, following cleavage of the N-terminal signal peptide, the protein passes through an apparently membrane-free intermediate form and only appears in the outer membrane after a delay of at least 50 seconds, following the completion of synthesis. From these results, we propose that the assembly of TonA involves translocation (with concomitant cleavage of the signal sequence) directly into the periplasm, followed by partitioning into the outer membrane. We further propose that the C terminus of TonA is essential for final maturation in the outer membrane in Sarkosyl-resistant form but that the C-terminal half of the molecule probably does not contain any topogenic sequences required for partitioning to the outer membrane.

Preparation of the FhuA (TonA) receptor protein from cell envelopes of an overproducing strain of Escherichia coli K-12

A rapid and simple method for purification of the FhuA receptor protein from cell envelopes of a FhuA-overproducing strain of Escherichia coli K-12 was developed. The overproduction of FhuA was programmed by the thermoamplifiable plasmid pHK232, which carried the fhuACD genes of pLC19-19 of the Clarke and Carbon collection. At low temperature (27 degrees C), pHK232 specified the overproduction of FhuA to levels comparable to those of major outer membrane proteins OmpF, OmpC, and OmpA. The amount of these proteins in the outer membrane was reduced along with overproduction of FhuA. Upon runaway replication of pHK232 at 37 degrees C, the precursor of the FhuA protein, proFhuA, was also accumulated in the cell envelope in amounts similar to FhuA. For extraction of the FhuA protein, crude cell envelopes were washed with 2% Triton X-100-6 M urea to remove less tightly bound proteins. Then FhuA but not proFhuA was solubilized by treating Triton X-100-urea-washed membranes with 1% octylglucoside-1 mM EDTA. This procedure yielded FhuA protein free from other membrane proteins. The amount of lipopolysaccharide and phospholipids was low and ranged from 5 to 15% and 10 to 25% of the weight of the FhuA protein, respectively. As shown by direct inactivation and by competition assays, the isolated FhuA protein retained receptor activity for ferrichrome, albomycin, colicin M, and phages T5 and T1.

Protein fusions of beta-galactosidase to the ferrichrome-iron receptor of Escherichia coli K-12

The fusion-generating phage lambda plac Mu1 was used to produce fusions of lacZ to fhuA, the gene encoding the ferrichrome-iron receptor (FhuA protein) in the outer membrane of Escherichia coli K-12. Fusions to the fhuA gene in a delta (lac) strain were selected by their resistance to bacteriophage phi 80 vir. Ten independent (fhuA'-'lacZ) fusions were all Lac+ and were resistant to the lethal agents which require the FhuA protein as receptor, i.e., phi 80 vir, T5, T1, UC-1, and colicin M; none could utilize ferrichrome as the sole iron source. Specialized transducing phages were obtained by illegitimate excision from the chromosome of each of the fusion-bearing strains, and EcoRI fragments which encoded the fusions were subcloned into the high-copy plasmid pMLB524. Physical mapping of the fusion-containing plasmids confirmed the presence of three restriction sites which were also located on the chromosomal DNA of sequences near the fhuA gene. The direction of transcription of the fhuA gene was deduced from the direction of transcription of the (fhuA'-'lacZ) gene fusion. Identification of the chimeric proteins was made by both radiolabeling cells and immunoprecipitating the LacZ-containing proteins with antibody to beta-galactosidase and by preparing whole cell extracts from Lac+ cells containing the cloned gene fusions. Two sizes of (FhuA'-'LacZ) proteins were detected, 121 kDa and 124 kDa. The DNA sequences at the unique fusion joints were determined. The sequence information allowed us to identify three distinct fusion joints which were grouped as follows, type I fusions, 5'-ACT GCT CAG CCA A-3'; type IIa fusions, 5'-GCG GTT GAA CCG A-3'; and type IIb fusions: 5'-ACC GCT GCA CCT G-3'. To orient these fhuA fusion joints, the complete nucleotide sequence of the fhuA gene was determined from a 2,902-base-pair fragment of DNA. A single open reading frame was found which translated into a 747-amino acid polypeptide. The signal sequence of 33 amino acids was followed by a mature protein with a molecular weight of 78,992. Alignment of the amino acid sequence of the FhuA protein with the amino acid sequences presented for two other tonB-dependent receptor proteins in the outer membrane of E. coli showed an area of local homology at the amino terminus of all three proteins.

Identification of polypeptides required for the export of haemolysin 2001 from E. coli

We have identified the polypeptides encoded by the haemolysin export genes from a haemolytic determinant 2001 carried by pLG570. This was previously cloned from an E. coli strain, serotype 04 isolated from a human urinary tract infection. Subclones from the recombinant plasmid pLG570 carrying hlyD analysed in vitro and in minicells showed that this gene is transcribed from an independent promoter and encodes a 53 Kd polypeptide. In contrast, detectable levels of the gene products encoded by hlyB were only observed when transcription presumably emanated from a vector promoter. This gene was found to encode at least two polypeptides apparently expressed from alternative translational start sites within a single reading frame. In minicells the major product was a 66 Kd polypeptide whilst after expression in vitro the major product was a 46 Kd polypeptide. Transposon mutagenesis leading to the synthesis of the expected truncated polypeptides was used to confirm the identity of the hlyD and the two hlyB products. Preliminary results suggest that the majority of the 53 Kd polypeptide is located in the inner membrane when cell envelopes from minicells and maxicells were fractionated using sarkosyl, although residual amounts of the 53 Kd polypeptide were also found in the outer membrane.

Overlapping of the coding regions for alpha and gamma components of penicillin-binding protein 1 b in Escherichia coli

The mode of biosynthesis of penicillin-binding protein(PBP)-1b in Escherichia coli was investigated by use of the plasmid carrying the ponB(PBP-1b) gene region. Analyses of the products synthesized in minicells and in vitro showed that PBP-1b was synthesized as two molecular species corresponding to the alpha and gamma components of PBP-1b. The coding regions for the alpha and gamma components were located within the ca. 3.7 kb MluI-HincII fragment and transcribed in the direction from the HincII to the MluI site. The capacity for producing the alpha component was abolished by a deletion extending to the MluI site ca. 0.7 kb inward from the HincII end of the ca. 3.7 kb fragment; the remaining 3.0 kb region with the MluI site at both ends directed the production of the gamma component alone. The production of the gamma component was enough to correct all the known defects caused by a ponB mutation. In addition to these results, the analyses for cross-reacting materials produced in correspondence to the various deletions indicated that the coding regions for the alpha and gamma components overlapped and that the N-terminal portion was responsible for the difference between the two components. The distal region about 0.7 kb long inward from the MluI end of the MluI-HincII fragment was dispensable for producing the functional PBP-1b, although the PBP-1b produced was curtailed. By a larger distal deletion reaching almost to the middle of the MluI-HincII fragment, the polypeptide produced for PBP-1b lost the ability to bind penicillin and still retained a low but significant activity for glycan synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic and biochemical characterization of the Escherichia coli K-12 fhuB mutation

The fhuB region of Escherichia coli K-12 was subcloned from pLC4-44 into pP lac to obtain pCPN1. Deletions of this recombinant plasmid were made, and a 1.4-kilobase PstI fragment was further subcloned into the vector plasmid pKK177-2 to obtain pCPN12. The response of tonA and tonB strains and fhuB strains containing the plasmids to 15 hydroxamate siderophores were assayed. Results showed that tonA strains were deficient only in the utilization of ferrichrome-type siderophores, whereas fhuB strains were deficient in the utilization of all hydroxamate-type siderophores. The response of the plasmid-containing fhuB strains to the siderophores showed that the fhuB gene resides on a 1.4-kilobase PstI fragment of DNA. The proteins synthesized by these plasmids were examined in maxicells of strain CSR603. Plasmid pCPN1 expressed five proteins of molecular weights 78,000, 40,000, 30,000, 24,000, and 13,700. By the use of deletions of pCPN1, the approximate order of the genes for these proteins was determined. Plasmid pCPN12 expressed no proteins other than the beta-lactamase proteins in maxicell strain CSR603. However, in maxicell strain BN660, a lon mutant, it expressed a 20,000-molecular-weight protein. Inner membrane vesicles made from tonB and fhuB strains were able to transport [55Fe]ferrichrome and [55Fe]rhodotorulate at rates similar to those obtained in vesicles from tonB+ and fhuB+ strains.

Molecular cloning of the ferrichrome-iron receptor of Escherichia coli K-12

A receptor protein in the outer membrane of Escherichia coli K-12 is required for the binding of ferrichrome-iron at the cell surface and for the transport of iron from this complex into the cell. This protein of Mr 78,000 is the product of the fhuA (previously called tonA) gene, located at 3.5 min on the E. coli chromosome. We cloned the fhuA gene into plasmid p343, a high-copy-number cosmid derived from pBR322. An 8.5-kilobase pair fragment of E. coli chromosomal DNA, generated by hydrolysis with the restriction endonuclease HindIII, was found to have conferred the FhuA+ phenotype to E. coli P8, which lacks the ferrichrome-iron receptor. A partial physical map of this recombinant plasmid pPM18 was established by determining the restriction endonuclease sites for BglII, EcoRI, PstI, PvuII, SmaI, and XhoI. The fhuA gene was localized to a 3.5-kilobase pair fragment of DNA whose extremities were defined by the restriction sites PstI-PvuII. A 7.5-fold enhancement of the rate of transport of iron from the ferrichrome complex was measured for cells which contained pPM18 as compared to wild-type E. coli K-12. Overproduction of the FhuA protein was detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of outer membrane proteins of the recombinant plasmid-containing strain. Proteins encoded by the subcloned DNA fragments were identified by [35S]methionine labeling of maxicells of E. coli CSR603, which contained recombinant plasmids; only one polypeptide chain, the presumptive fhuA gene product, was detected.

Cloning and expression of the fhu genes involved in iron(III)-hydroxamate uptake by Escherichia coli

Each of the four hydroxamate compounds, ferrichrome, aerobactin, rhodotorulic acid, and coprogen, known to transport ferric iron into Escherichia coli requires a specific outer membrane receptor protein. In addition, common transport functions for all four ferric hydroxamate compounds have been identified in the 3.5-min region of the linkage map and designated fhu. The fhu region was cloned into pBR322. By restriction analysis, Tn5 insertion mutations, and complementation studies between plasmid fragments and chromosomal mutants at least four loci in the order fhuA fhuC fhuD fhuB were found. The genetic products were determined in maxicells and minicells. fhuA codes for the known 78,000-dalton receptor protein and the 81,000-dalton precursor in the outer membrane, fhuC codes for a 30,000-dalton protein, and fhuD encodes a 26,000-dalton protein in the cytoplasmic membrane. No protein(s) could be assigned to the fhuB region. Truncated proteins derived from partial fhuA genes (68,000, 42,000, and 39,000 daltons) and a partial fhuD gene (24,000 daltons) and the strong polar effect on the expression of the genes indicated the direction of transcription to be from fhuA to fhuD.

Integration of the overproduced bacteriophage T5 receptor protein in the outer membrane of Escherichia coli

The tonA gene codes for an outer membrane protein, a receptor of phage T5, the TonA protein. Strains harboring pLG513, a multicopy plasmid in which the tonA gene has been cloned, overproduced TonA protein, which appeared in sodium dodecyl sulfate-polyacrylamide gel electrophoresis of cell envelope proteins as a 78,000-molecular-weight protein. Identical results have been observed by Plastow et al. (FEBS Lett. 131:262-264, 1981) with plasmid pLC19-19, in which the tonA gene has also been cloned. The activity of the TonA protein, measured by its capacity to inactivate phage T5, increased by five- to sixfold in purified envelopes of cells harboring pLG513 compared with cells lacking the plasmid. Solubilization of the cytoplasmic membrane by Triton-Mg2+ treatment did not increase this activity. However, partial solubilization of outer membrane proteins by Triton-EDTA unmasked further T5 receptor activity, resulting in a final increase of around 50-fold, a value more consistent with the expected gene dosage effect. Treatment of whole cells by trypsin in conditions in which trypsin is allowed to enter the outer membrane revealed that part of the overproduced T5 receptors were embedded in the outer membrane and masked by a trypsin-sensitive protein. In addition, no T5 receptor activity was detected in either the periplasmic space or the cytoplasm. These results suggest that all of the overproduced TonA molecules were synthesized in an active form and integrated in the outer membrane, but only a small fraction could be reached or recognized by phage T5 in vivo.