Nucleomorphs as genomic tools for photosynthesis research

Abstract

Beside the nucleus, the plastid and mitochondria, cryptomonads possess a fourth DNA-containing semiautonomous organelle, the nucleomorph. This organell was shown to be the remnant of the nucleus of a former free-living rhodophytic alga, which was engulfed and reduced to a complex plastid by a heterotrophic host cell. By sequencing the three minute nucleomorph chromosomes of the cryptomonad Guillardia theta, we present evidence that the purpose of the nucleomorph is to perpetuate itself, the periplastid space it is embedded in and a eukaryotic expression machinery for the synthesis of 30 nucleomorph-encoded chloroplast proteins (Douglas et al. (2001), Nature 410, 1091-1096). Some of these chloroplast located proteins have homologous sequences in the cyanobacterium Synechocystis sp. PCC6803 but differ from those by an amino-terminal extension which functions as a transit peptide for the translocation to the plastid. Beside proteins of known function (FtsZ, Hlip, CbbX), we identified 11 gene products whose homologs in cyanobacteria do not have any known function. These are the genes of interest we have in part characterized. By knock-out experiments in cyanobacteria and physiological investigations we analyzed the functions of the nucleomorph-encoded proteins in respect to the photosynthesis machinery. Major results so far are the detection of components attached to photosystem II and the creation of mutants with a reduced phycocyanin content.