Bacteriophage T4 baseplate components. II. Binding and location of bacteriophage-induced dihydrofolate reductase

An immunoblot assay reveals that bacteriophage T4 thymidylate synthase and dihydrofolate reductase are not virion proteins

Numerous reports describe the phage T4 enzymes thymidylate synthase and dihydrofolate reductase as structural components of the baseplate. However, Y. Wang and C. K. Mathews (J. Virol. 63:4736-4743, 1989) reported that antisera against the respective recombinant enzymes failed to neutralize phage infectivity, in contrast to previous results. Moreover, a deletion mutant lacking the genes for these two enzymes adsorbed normally to host cells. Since these findings tended to undermine the idea of the two enzymes as structural proteins, we developed a quantitative immunoblot assay to resolve the issue directly. Our results show that both enzymes are present only as minor contaminants (< 0.05 copy per phage) and as such cannot be bona fide structural proteins.

Analysis of T4 bacteriophage deletion mutants that lack td and frd genes

The roles of bacteriophage T4-encoded thymidylate synthase and dihydrofolate reductase as virion structural components have been further investigated. Two mutants, del(63-32)7 and del(63-32)9, bearing deletions in the gene 63 to 32 region of the T4 genome, were characterized by Southern blotting analysis, as well as by enzyme and immunological assays. Our results have confirmed the original report of Homyk and Weil (Virology 61:505-523, 1974) that del7 and del9 each carries a deletion of about 4.0 kilobases, which totally eliminates the frd gene, encoding dihydrofolate reductase, and the td gene, encoding thymidylate synthase. With the well-characterized deletion mutants, along with newly prepared antisera against T4-encoded thymidylate synthase and dihydrofolate reductase, we have reevaluated the experimental results supporting the idea that T4-induced dihydrofolate reductase and thymidylate synthase are essential T4 baseplate components and antigenic determinants of phage particles. These deletion mutant phages are not targets for neutralization by antisera against either dihydrofolate reductase or thymidylate synthase purified from cloned genes. Furthermore, these newly prepared antisera also cannot neutralize the infectivity of T4D. Those results suggest that the phage-neutralizing components in the old antisera used in the earlier studies were not antibodies against either dihydrofolate reductase or thymidylate synthase but were antibodies against minor components of the purified enzyme preparations. Study of the biological properties of the deletion mutants indicates that T4-induced thymidylate synthase and dihydrofolate reductase play significant roles in growth of the phage beyond their known roles in nucleotide biosynthesis, even though they are apparently not essential for phage viability. The deletion mutants should be useful in defining these roles.

Attachment of tail fibers in bacteriophage T4 assembly. Identification of the baseplate protein to which tail fibers attach

A phage-neutralizing rabbit antiserum collected after immunization with tail-fiberless bacteriophage T4 particles was adsorbed with complete T4 phage. The resulting adsorbed serum inhibited tail fiber attachment in vitro. To identify the antigens against which this inhibitory activity was directed, blocking experiments were carried out with the adsorbed serum. Isolated complete baseplates and mutant-infected-cell extracts lacking known baseplate gene products but containing gene 9 product showed similar high levels of blocking activity. By contrast, both tail-fiberless particles lacking gene 9 product and infected-cell extracts made with gene 9 mutants showed 30-fold to 100-fold lower blocking activity. These results strongly support the conclusion that gene 9 product is the baseplate protein to which tail fibers attach.

Purification and some properties of bacteriophage T4 particle-associated lysozyme

Bacteriophage T4 particle-associated lysozyme, purified to electrophoretic homogeneity, was found to be a protein with a relative molecular mass of 15000. The lysozyme was purified from the particles of bacteriophage T4 e mutant and from the lysates of the 5tsl e T4 mutant, in which the enzyme is in soluble form. In the purification procedure advantage was taken of the affinity of the enzyme for GlcNAc-MurNac-LAla-DGlu-msA2pm-DAla (C6 muropeptide), one of the products of the digestion of Escherichia coli murein with lysozyme. The test for the quick estimation of bacteriolytic activity of the enzyme, using E. coli B freeze-dried cells, is described. The pH optimum of the particle-associated lysozyme was equal to about 6.0, ionic strength optimum to 0.05-0.1 M, and optimum Triton X-100 concentration to 1%, when this substrate was used. Some of the aspects of the possible biological significance of the particle-associated lysozyme in bacteriophage T4 infection are discussed.

Structural role of the polyglutamate portion of the folate found in T4D bacteriophage baseplate

Three types of reagents were used to determine the structural role and location of the polyglutamate portion of the Escherichia coli T4D bacteriophage baseplate dihydropteroyl hexaglutamate. These reagents were examined for their effect in vitro on some of the final steps in phage baseplate morphogenesis. The reagents were (i) a series of oligopeptides composed solely of glutamic acid residues but with various chemical linkages and chain lengths; (ii) a homogeneous preparation of carboxypeptidase G1, an exopeptidase that hydrolyzes carboxyl-terminal glutamates (or aspartates) from simple oligopeptides, including the gamma-glutamyl bonds on folyl polyglutamates as well as the bond between the carboxyl group of the p-aminobenzoyl moiety and the amino group of the first glutamic acid residue of folic acid; and (iii) antisera prepared against a polyglutamate hapten. All three types of reagent markedly inhibited the attachment of the phage long tail fibers to the baseplate. Other steps in baseplate assembly such as the addition of T4D gene 11 or gene 12 products were not affected by any of these reagents. These results indicate that the polyglutamate portion of the folate is located near the attachment site on the bacteriophage baseplate for the long tail fibers.

Bacteriophage T4-coded dihydrofolate reductase: synthesis, turnover, and location of the virion protein

Dihydrofolate reductase plays a dual role in bacteriophage T4, first, as an enzyme of thymidylate metabolism, and second, as a protein component of the tail baseplate. Antibody to the purified enzyme has been used to study its synthesis and intracellular turnover. The antibody specifically precipitates one protein from T4D-infected cell extracts. This has been identified as dihydrofolate reductase, although the polypeptide molecular weight (22,000) is lower than that earlier determined for this enzyme. The protein comigrates on gels with pY, a genetically undefined protein component of the baseplate. However, it is not pY, for pY is synthesized late in infection, whereas virtually no dihydrofolate reductase synthesis occurs later than 10 min after infection at 37 degrees C. Dihydrofolate reductase, once formed, is neither degraded nor converted to proteins of higher or lower molecular weight. Thus, it is probably incorporated into virions at the same molecular weight as that of the soluble enzyme. 125I-radiolabeled antibody binds to the wedge substructure of the baseplate, and this binding is blocked by preincubation with purified T4 dihydrofolate reductase. Thus, the enzyme protein seems to be a component of the wedge.

Folate polyglutamates in T4D bacteriophage and T4D-infected Escherichia coli

The folate compound which is a structural component of the Escherichia coli T-even bacteriophage baseplates, has been identified as the hexaglutamyl form of folic acid using a new chromatographic procedure (Baugh, C.M., Braverman, E. and Nair, M.G. (1974) Biochemistry 13, 4952-4957). It has also been found that the host cell contains a variety of polyglutamyl forms of folic acid. The major form is the triglutamate (about 50%) but small amounts of higher molecular weight folates including the octaglutamate (1.8%) have been identified. Upon infection with wild-type T4D bacteriophage there is a shift in the distribution of the folate compounds so that the folyl polyglutamyl compounds having the higher molecular weights are increased. Infection of E. coli with baseplate mutants of T4D containing an amber mutation in gene 28 resulted in the formation of significant amounts (over 7%) of folate compound(s) of molecular weight much higher than those observed either in uninfected cells or cells infected with wild-type T4D. It is suggested that the T4D gene 28 product functions to cleave glutamate residues from high molecular weight folyl polyglutamates to increase the availability of the folyl hexaglutamate for virus assembly.

Bacteriophage T4 virion baseplate thymidylate synthetase and dihydrofolate reductase

Additional evidence is presented that both the phage T4D-induced thymidylate synthetase (gp td) and the T4D-induced dihydrofolate reductase (gp frd) are baseplate structural components. With regard to phage td it has been found that: (i) low levels of thymidylate synthetase activity were present in highly purified preparations of T4D ghost particles produced after infection with td(+), whereas particles produced after infection with td(-) had no measurable enzymatic activity; (ii) a mutation of the T4D td gene from td(ts) to td(+) simultaneously produced a heat-stable thymidylate synthetase enzyme and heat-stable phage particles (it should be noted that the phage baseplate structure determines heat lability); (iii) a recombinant of two T4D mutants constructed containing both td(ts) and frd(ts) genes produced particles whose physical properties indicate that these two molecules physically interact in the baseplate. With regard to phage frd it has been found that two spontaneous revertants each of two different T4D frd(ts) mutants to frd(+) not only produced altered dihydrofolate reductases but also formed phage particles with heat sensitivities different from their parents. Properties of T4D particles produced after infection with parental T4D mutants presumed to have a deletion of the td gene and/or the frd gene indicate that these particles still retain some characteristics associated with the presence of both the td and the frd molecules. Furthermore, the particles produced by the deletion mutants have been found to be physically different from the parent particles.

Bacteriophage T4 virion dihydrofolate reductase: approaches to quantitation and assessment of function

This paper is concerned with the physiological role(s) of T4 phage-coded dihydrofolate reductase, which functions both in DNA precursor metabolism and as a virion protein. (i) We have detected enzyme activity in noninfectious particles produced under restrictive conditions by gene 11 mutants. This supports the conclusion of Kozloff et al. (J. Virol. 16:1401-1408, 1975) that the protein lies in the baseplate, covered by the gene 11 protein. (ii) We have obtained further evidence for virion dihydrofolate reductase as the target for neutralizing activity of T4 dihydrofolate reductase antiserum and as a determinant of the heat lability of the virion. This derives from our observation that the reductases specified by T4B and T4D differ in several properties. (iii) We have investigated several anomalous properties of T4 mutants bearing deletions that reportedly extend into or through the frd gene, which codes for dihydrofolate reductase. Evidence is presented that the deletions in fact do not extend through frd. These strains direct the synthesis of material that cross-reacts with antiserum to homogeneous dihydrofolate reductase. Moreover, they are all quite sensitive to the phage-neutralizing effects of this antiserum. In addition, they are restricted by several of the hospital strains, wild-type strains of Escherichia coli supplied by the California Institute of Technology group. (iv) We have attempted to detect dihydrofolate reductase among early-synthesized proteins present in T4 tails. Two such proteins are seen, one of which is evidently the gene 25 product and one that is a bacterial protein. Quantitation of our electrophoretic technique has allowed determination of the number of molecules of some T4 tail components present per virion. (v) Finally, we have compared the T4 dihydrofolate reductase with the corresponding enzyme specified by two plasmids conferring resistance to trimethoprim (Skold and Widh, J. Biol. Chem. 249:4324-4325, 1974). Although the enzymes are similar in some properties, they differ in several important respects, including immunological activity.

Bacteriophage T4 baseplate components. III. Location and properties of the bacteriophage structural thymidylate synthetase

Two T4D thymidylate synthetase (td) temperature-sensitive mutants have been isolated and characterized. Both mutants produce heat-labile phage particles. This observation supports the view that this viral-induced protein is a phage structural component. Further, antiserum to td has been shown to block a specific step in tail plate morphogenesis. The results indicated that the td protein is largely covered by the T4D tail plate gene 11 protein. Since the phageinduced dihydrofolate reductase (dfr) also is partially covered by the gene 11 protein, it appears that td was adjacent to the tail plate dfr. This location has been confirmed by constructing a T4D mutant which is dfrtstdts and showing that these two tail plate constituents interact and give altered physical properties to the phage particles produced. A structural relationship for the tail plate folate, dfr, and td has been reported.

Bacteriophage T4 baseplate components. I. Binding and location of the folic acid

Two different proteins with high affinities for the pteridine ring of folic acid have been used to determine the location of this portion of the folate molecule in the tail plate of T4D and other T-even bacteriophage particles. The two proteins used were (i) antibody specific for folic acid and (ii) the folate-binding protein from bovine milk. Both proteins were examined for their effect on various intact and incomplete phage particles. Intact T2H was weakly inactivated by the antiserum but not by the milk protein. No other intact T-even phage, including T4D, was affected by these two proteins. When incomplete T4D particles were exposed in an in vitro morphogenesis system, it was found that neither of the two proteins affected either the addition of the long tail fibers to fiberless particles or the addition of tail cores to tail plates. On the other hand, these two proteins specifically blocked the addition of T4D gene 11 product to the bottom of T4D baseplates. After the addition of the gene 11 protein, these two reagents did not inhibit the further addition of the gene 12 protein to the baseplate. It can be concluded that the phage folic acid is a tightly bound baseplate constituent and that the pteridine portion of the folic acid is largely covered by the gene 11 protein.