Cloning and characterization of the nuclear AC115 gene of Chlamydomonas reinhardtii

The nuclear ac115 mutant of Chlamydomonas reinhardtii is specifically block ed in the synthesis of the chloroplast encoded D2 protein of the photosyste m II reaction center at a point after translation initiation. Here, we repo rt the identification of the AC115 gene through complementation rescue of t he ac115 mutant strain, using an indexed cosmid library of Chlamydomonas ge nomic DNA. AC115 is a small, novel, intronless nuclear gene which encodes a protein of 113 amino acids. The amino terminal end of the Ac115 protein is rich in basic amino acids and has features which resemble a chloroplast tr ansit sequence. A hydrophobic stretch of amino acids at the protein's carbo xyl terminus is sufficiently large to be a membrane spanning or a protein/p rotein interaction domain. Various models are discussed to account for the mechanism by which Ac115p works in D2 synthesis. The ac115 mutant allele wa s sequenced and determined to be an A-to-T transversion at the first positi on of the fourth codon of the coding sequence. This mutation changes an AAG codon to a TAG nonsense codon and results in a null phenotype.