Induction of E. coli recA protein via recBC and alternate pathways: quantitation by enzyme-linked immunosorbent assay (ELISA)

Summary An enzyme-linked immunosorbent assay (ELISA) has been adapted to measure E. coli recA protein in the 1 to 10 ng range in whole-cell sonicates, membrane extracts, and osmotic shock fluid from 2x10⁸ cells. The specific activity of recA protein is maintained at a relatively constant basal level (800 to 1,200 molecules per cell for wild-type E. coli in L-broth, salt-depleted broth and minimal media) during early-log and mid-log phase growth, but it increases by two- to ten-fold as the culture approaches saturation density. Nalidixate-induced levels are 20- to 50-fold higher, and 100-fold higher in a constitutive tif^-spr^-mutant.Induction of recA protein synthesis by nalidixic acid, which normally requires functional recBC enzyme, also occurs in recB^-and recC^-cells by pathways activated by mutation in the sbcA and sbcB indirect suppressors. In recB^-sbcA^-mutants, exonuclease VIII, the recE gene product, is required for induction of recA protein. Abolition of exonuclease I activity by mutation in sbcB allows induction of recA protein by nalidixate in recB^-and recC^-cells. Mutation in recF does not affect induction by nalidixate in RecBC⁺ cells, but it enables induction to occur in RecBC^- cells, suggesting that recF gene product is involved in regulation of recA protein.