Modulation of the heat shock and stringent responses of Escherichia coli by a mutation in theglyA gene.

Abstract

A genetic screen, designed to isolate mutants of Escherichia coli altered in the ability to induce the heat shock response, identified a strain unable to induce the heat shock proteins in a defined medium lacking methionine after exposure to 2,4-dinitrophenol (DNP). The strain also exhibited a relaxed-like phenotype, i.e. it continued to synthesize translational machinery rapidly during the period of inhibited growth following exposure to DNP, whereas wild-type cells decreased the rate at which the translational apparatus was made. Also, this strain also grew slowly at 28$\sp\circ$C, and linearly at 42$\sp\circ$C. The mutation responsible for this phenotype mapped to the glyA gene, a biosynthetic gene encoding the enzyme that converts serine and tetrahydrofolate to glycine and 5,10-methylene-tetrahydrofolate. This reaction is the major source of glycine and one-carbon units in the cell. The abnormal phenotypes associated with the glyA mutation were reversed when methionine was included in the growth medium. Because fixed one-carbon units, as methionine, allowed mutant cells to heat shock after exposure to DNP, a one-carbon restriction was considered possibly responsible for the phenotypes described above. It was postulated that the relaxed-like phenotype arose from a partial block in translational initiation, resulting from low levels of formylated initiator tRNA, and that the abnormal heat shock was a result of changes in physiology caused by the relaxed-like state. Measurements of f-met-tRNA levels in the glyA mutant were consistent with this hypothesis. However, an fmt mutant, which is unable to formylate initiator tRNA, failed to mimic the glyA mutant in these respects, suggesting that the phenotype associated with the glyA mutant is not the result of a low level of f-met-tRNA. In addition, a $\Delta relA\Delta spoT$ mutant failed to exhibit either the relaxed-like or abnormal heat shock phenotypes. This suggests that the stringent response seen following DNP addition may arise via a ppGpp independent mechanism. The results with this glyA mutant establish a nutritional modulation of the heat shock response that is connected with a novel form of the stringent response network.