In vitro random mutagenesis of the D1 protein of the photosystem II reaction center confers phototolerance on the cyanobacterium Synechocystis sp. PCC 6803.
The D1 protein of the photosystem II reaction center is thought to be the most light-sensitive component of the photosynthetic machinery. To understand the mechanisms underlying the light sensitivity of D1, we performed in vitro random mutagenesis of the psbA gene that codes for D1, transformed the unicellular cyanobacterium Synechocystis sp. PCC 6803 with mutated psbA, and selected phototolerant transformants that did not bleach in high intensity light. A region of psbA2 coding for 178 amino acids of the carboxyl-terminal portion of the peptide was subjected to random mutagenesis by low fidelity polymerase chain reaction amplification or by hydroxylamine treatment. This region contains the binding sites for Q(B), D2 (through Fe), and P680. Eighteen phototolerant mutants with single and multiple amino acid substitutions were selected from a half million transformants exposed to white light at 320 micromol m(-2) s(-1). A strain transformed with non-mutagenized psbA2 became bleached under the same conditions. Site-directed mutagenesis has confirmed that one or more substitutions of amino acids at residues 234, 254, 260, 267, 322, 326, and 328 confers phototolerance. The rate of degradation of D1 protein was not appreciably affected by the mutations. Reduced bleaching of mutant cyanobacterial cells may result from continued buildup of photosynthetic pigment systems caused by changes in redox signals originating from D1.