Lack of a functional plastid tRNACys gene is associated with loss of photosynthesis in a lineage of parasitic plants

Abstract

We recently reported that the gene for chloroplast tRNA(Cys)(GCA) is a pseudogene in the plastid DNA of Epifagus virginiana, a non-photosynthetic parasitic flowering plant in the family Orobanchaceae. Since this is the only tRNA(Cys) gene in the plastid genome, and since Epifagus appears to possess a functional plastid translational apparatus, it seems probable that nuclear-encoded tRNAs are imported into plastids to effect translation. In this study we have surveyed species closely related to Epifagus to establish how widespread the loss of this tRNA gene has been. We find that Conopholis americana, another non-photosynthetic parasite, lacks the gene altogether, but that seven closely-related photosynthetic plants (both parasitic and free-living) maintain an intact chloroplast tRNA(Cys) gene. Thus, the tRNA(Cys) gene appears to have become non-functional at the same time that photosynthetic ability was lost. This may be because the levels of putatively imported tRNAs are sufficient to meet the demands of plastid gene expression under nonphotosynthetic conditions only.