Properties of a Revertant of Escherichia coli Viable in the Presence of Spermidine Accumulation: Increase in l- Glycerol 3-Phosphate

Abstract

ABSTRACT Escherichia coli CAG2242 cells are deficient in the speG gene encoding spermidine acetyltransferase. When these cells were cultured in the presence of 0.5 to 4 mM spermidine, their viability was greatly decreased through the inhibition of protein synthesis by overaccumulation of spermidine. When the cells were cultured with a high concentration of spermidine (4 mM), a revertant strain was obtained. We found that a 55-kDa protein, glycerol kinase, was overexpressed in the revertant and that synthesis of a ribosome modulation factor and the RNA polymerase ς 38 subunit, factors important for cell viability, was increased in the revertant. Levels of l -glycerol 3-phosphate also increased in the revertant. Transformation of glpFK , which encodes a glycerol diffusion facilitator ( glpF ) and glycerol kinase ( glpK ), to E. coli CAG2242 partially prevented the cell death caused by accumulation of spermidine. It was also found that l -glycerol 3-phosphate inhibited spermidine binding to ribosomes and attenuated the inhibition of protein synthesis caused by high concentrations of spermidine. These results indicate that l -glycerol 3-phosphate reduces the binding of excess amounts of spermidine to ribosomes so that protein synthesis is recovered.