pH-dependent autocleavage of lambda repressor occurs in the operator-bound form: characterization of lambda repressor autocleavage Available to Purchase

The first-order rate constants for the RecA-independent, spontaneous, pH-dependent autocleavage of the λ cI repressor was measured in the present study at pH 10.6 at 27, 37 and 42 °C respectively. Autocleavage of the repressor occurs also at pH 9 and 8, although at progressively slower rates. We demonstrate that the spontaneous autocleavage occurs also in the operator-bound state, at a rate either higher than or equal to the rate in solution, depending on the pH value. Owing to the near equality of the rate constant in both operator-free and operator-bound repressors, it can be inferred that the cleavage site has a similar structure and dynamics with respect to the catalytic site in both forms at neutral pH. Covalent modification using PMSF, brought about by a large molar excess of the reagent, inhibits autocleavage of the λ repressor. The difficulty in obtaining this covalent modification is rationalized using our recent λ repressor models. Bimolecular type II trans-cleavage was observed previously for mutant LexA repressors lacking a crucial catalytic serine or lysine residue [Kim and Little (1993) Cell (Cambridge, Mass.) 73, 1165–1173], but it could still be cleaved by an 85–202 ‘enzyme’ fragment possessing an improved or hypercleavable character lacking its own cleavage site. Such a type II trans-cleavage was not observed with the covalently modified intact λ repressor used as substrate and the purified wild-type λ repressor 112–236 fragment used as the ‘enzyme’. All these results show that for the wild-type λ repressor, the catalytic site is close to the cleavage site in both operator-free and -bound states. In the lytic pathway, the repressor is mainly cleaved via RecA-mediated cleavage, which occurs much faster than the spontaneous autocleavage; the possible biological significance of this slow, spontaneous, but constant, autocleavage is related to the lysogenic state, when RecA-mediated cleavage is absent.