MUTATIONS IN A SEQUENCE NEAR THE N-TERMINUS OF THE SMALL-SUBUNIT ALTER THE CO2 O-2 SPECIFICITY FACTOR FOR RIBULOSE-BISPHOSPHATE CARBOXYLASE/OXYGENASE/

Seven unique substitutions have been introduced by site-directed mutag enesis into the first conserved region of the small subunit of ribulos e bisphosphate carboxylase/oxygenase from Anacystis nidulans 6301. Aft er expression of each large, altered-small subunit gene tandem in Esch erichia coli, two substitutions in the small subunit tyr17-->asp17 (Y1 7D) and arg10-->gly10(R10G) result in little or no carboxylase activit y. For the latter substitution, no L8S8 enzyme complex could be detect ed suggesting that this mutation prevents assembly. Mutant enzymes con taining the following substitutions R11G, T14A, S16A, Y17D and P19A ha ve CO2/O-2 specificity factors (tau values) of 40, 35, 18, 39 and 44, respectively, compared with that of 44 for wild-type recombinant enzym e whereas P20A has full carboxylase activity and a tau value of 55.