Growth properties of a folA null mutant of Escherichia coli K12.

In Escherichia coli, dihydrofolate reductase is required for both the de novo synthesis of tetrahydrofolate and the recycling of dihydrofolate produced during the synthesis of thymidylate. The coding region of the dihydrofolate reductase gene, folA, was replaced with a kanamycin resistance determinant. Unlike earlier deletions, this mutation did not disrupt flanking genes. When the mutation was transferred into a wild-type strain and a thymidine-(thy) requiring strain, the resulting strains were viable but slow growing on rich medium. Both synthesized less folate than their parents, as judged by the incorporation of radioactive para-aminobenzoic acid. The derivative of the wild-type strain did not grow on any defined minimal media tested. In contrast, the derivative of the thy-requiring strain grew slowly on minimal medium with thy but exhibited auxotrophies on some combinations of supplements. These results suggest that when folates are limited, they can be distributed appropriately to folate-dependent biosynthetic reactions only under some conditions.

Measurement of the uptake of radioactive para-aminobenzoic acid monitors folate biosynthesis in Escherichia coli K-12.

This study was undertaken to develop a method for rapidly and easily estimating the folate content of different strains of Escherichia coli. Cells were grown to stationary phase in medium containing radioactive para-aminobenzoic acid. The amount of label incorporated into cells was measured by collecting the cells on a filter, washing, and then determining the radioactivity retained on the filter. The addition of unlabeled para-aminobenzoic acid or the antifolate drugs sulfathiazole or trimethoprim reduced the uptake of the radioactive compound. These results indicate that uptake measurements monitored folate biosynthesis. The assay is well suited for the analysis of large numbers of samples and does not require specialized equipment.

Changes in translational accuracy of Escherichia coli when folate metabolism is perturbed.

Translational accuracy was monitored in Escherichia coli mutants which contain abnormal folate pools. A decrease in translational accuracy was indicated by the increased production of a T4 phage mutant containing a UGA mutation in a tail fibre gene. An E. coli folC mutant suppressed the phage mutant under conditions where it presumably accumulated methyl-tetrahydrofolate (methyl-THF). A UGA suppressor strain with the mutation affecting the primary structure of the tRNA(trp) normally suppressed the phage mutant. When the strain was made thymine-requiring it no longer suppressed. The accumulation of methyl-THF which permits suppression by thymine-requiring strains may act to interfere with suppression by the tRNA suppressor, possibly by changing the modification pattern of the tRNA.

Kasugamycin inhibition of nonsense suppression by thymine-requiring strains of Escherichia coli K12.

Thymine-requiring strains of Escherichia coli suppress nonsense and frame-shift mutations. This appears to occur during translation, suggesting that the lack of activity of an enzyme thymidylate synthase, required for the synthesis of a DNA precursor, alters the fidelity of translation. The aminoglycoside antibiotic kasugamycin, which enhances translational accuracy in vitro, prevents thymine-requiring cells from suppressing. The inhibition of suppression by kasugamycin is not prevented by the introduction of two different kasugamycin-resistance mutations, although the dose required for inhibition increases. These observations support the conclusion that suppression occurs during translation.

Nonsense suppression in thymine-requiring strains of Escherichia coli is a consequence of altered folate metabolism.

Thymine-requiring strains of Escherichia coli suppress nonsense and frameshift mutants of T4 phage. We proposed that these mutants make errors during translation because of an imbalance in folate metabolism. A thymine-requiring strain grown under suppressing conditions has elevated levels of reduced folates. We tested the effect of either mutational blocks or the inhibition of various steps in folate biosynthesis on suppression. Conditions which prevent the accumulation of 5-methyl tetrahydrofolate inhibit suppression, suggesting that elevated levels of this folate are required for suppression. Furthermore, conditions that result in an accumulation in dihydrofolate inhibit suppression.

Modification of the suppressor phenotype of thymine requiring strains of Escherichia coli.

Thymine requiring strains of Escherichia coli suppress nonsense and frameshift mutations during translation. Strains with different genetic backgrounds exhibited different nonsense suppression spectra and showed differences in the apparent suppression efficiency. Part of this strain difference is due to a presumably novel gene (tsmA) mapping near 39 min. This gene affects the spectrum and apparent efficiency of suppression, and appears to affect the utilization of thymidine.

Frameshift suppression by thyA mutants of Escherichia coli K-12.

We have extended our previous study on the suppression of frameshift mutants by Escherichia coli thyA mutants by assaying suppression of 15 rIIB frameshift mutants of bacteriophage T4 on one of our suppressing thyA mutant strains. The majority of insertion mutants were suppressible, whereas none of the deletion mutants tested was suppressible. Frameshift suppression could be inhibited by adding thymidine to the assay medium, but was not affected by the presence of a restrictive rpsL mutation in the host strain. We suggest that the frameshift suppression event occurs at a nonsense codon generated by the frameshift mutation.

Neomycin is more efficient than streptomycin in suppressing frameshift mutations.

The effects of streptomycin and neomycin on the phenotypic suppression of frameshift mutations in the lacZ gene of Escherichia coli and on the efficiency of suppression of amber mutations in T4 phage by the informational supE tRNA nonsense suppressor were compared. Neomycin stimulated much more efficiently than streptomycin the phenotypic suppression of frameshift mutations. Because neomycin favors mismatches of the central codon base whereas streptomycin favors mismatches of the first codon base, this result suggests that mismatching of the central codon base pair and shifting of the reading frame are two correlated phenomena. In contrast, both streptomycin and neomycin stimulated about equally the efficiency of the tRNA nonsense suppressor, an effect probably related to their interference with the proofreading control in tRNA selection.

Suppression by thymidine-requiring mutants of Escherichia coli K-12.

Thymidine-requiring strains of Escherichia coli isolated by trimethoprim selection often simultaneously acquire the ability to suppress bacteriophage T4 nonsense mutations. Suppression is lost in Thy+ revertants and recombinants, but is sometimes retained in thyA plasmid-bearing transformants. Suppression is restricted in Strr derivatives of the Thy- mutants, indicating that suppression occurs at the level of translation.

Thymine inhibits suppression by an Escherichia coli nonsense and frameshift suppressor.

A mutant strain of Escherichia coli suppressed a frameshift and some UAG, UAA, and UGA mutants of bacteriophage T4 at 37 degrees C but not at 31 degrees C. This suppression was inhibited by the addition of thymine or thymidine to the medium used to test phage growth. Furthermore, the suppressor strain required thymine or thymidine for growth on minimal medium at 43 degrees C and if this auxotrophy was removed by reversion or recombination the strain no longer suppressed. These results suggest a link between thymidine nucleotide biosynthesis and suppression.

A single mutation affects L-serine deaminase, L-leucyl-, L-phenylalanyl-tRNA protein transferase, and proline oxidase activity in Escherichia coli K-12.

A mutation at a single locus, wyb, results in several phenotypic changes in Escherichia coli K-12. The Wyb- phenotype includes: (i) an increase in L-serine deaminase activity, together with a loss of inducibility by L-leucine; (ii) an absence of L-leucyl-, L-phenylalanyl-tRNA protein transferase activity; (iii) inducibility of proline oxidase by proline; and (iv) a loss of ability to use maltose as a carbon and energy source.

Genetic characterization of the temperature-sensitive and suppression phenotypes of Escherichia coli mutant N4316.

Escherichia coli mutant N4316 is temperature sensitive and exhibits temperature-dependent suppression. These phenotypes are due to separate genes, as shown by reversion and mapping studies. The suppressor mutation was mapped and lies near argF.

Bacteriophage transformation of PBS2 in Bacillus subtilis.

Transformation of temperature-sensitive mutants of bacteriophage PBS2 for Bacillus subtilis was demonstrated. The number of transformants was linearly related to the concentration of DNA within a range of 0.01 to 1 mug/ml. No transformants were obtained when the DNA was pretreated with DNase. PBS2 DNA sheared to approximately 1% of the total chromosome length was centrifuged in Cs2SO4-Hg gradients to fractionate the DNA according to the base composition. Transformation experiments carried out with the fractionated DNA indicated the possibility of determining the base composition of different regions of the phage chromosome.