A MUTATION IN THE SMALL-SUBUNIT OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE THAT REDUCES THE RATE OF ITS INCORPORATION INTO HOLOENZYME/

A mutant of the small subunit of ribulose-1,5-bisphosphate carboxylase /oxygenase (Rubisco), in which Arg53 is replaced by Glu, was synthesiz ed and imported into isolated chloroplasts. The mutant protein was eff iciently imported into the chloroplast and correctly processed to the mature size. Like the wild type protein, it was stable over a period o f at least 2 h. Unlike the wild-type protein however, most of the muta nt protein was not assembled with holo-Rubisco at the end of a 10-min import reaction. It migrated instead as a diffused band on a non-denat uring gel, slower than the precursor protein, but faster than the holo enzyme. The level of the unassembled mutant protein in the stroma decr eased with time, while its level in the assembled fraction has increas ed, indicating that this protein is a slowly-assembled, rather than a non-assembled, mutant of the small subunit of Rubisco. Accumulation of the mutant protein in the holoenzyme fraction was dependent on ATP an d light. The transient species, migrating faster than the holoenzyme b ut slower than the precursor protein, may represent an intermediate in the assembly process of the small subunit of Rubisco.