The chloroplast genome of the diatom Seminavis robusta: new features introduced through multiple mechanisms of horizontal gene transfer.

The chloroplasts of heterokont algae such as diatoms are the result of a secondary endosymbiosis event, in which a red alga was engulfed by a non-photosynthetic eukaryote. The diatom chloroplast genomes sequenced to date show a high degree of similarity, but some examples of gene replacement or introduction of genes through horizontal gene transfer are known. The evolutionary origin of the gene transfers is unclear. We have sequenced and characterised the complete chloroplast genome and a putatively chloroplast-associated plasmid of the pennate diatom Seminavis robusta. The chloroplast genome contains two introns, a feature that has not previously been found in diatoms. The group II intron of atpB appears to be recently transferred from a Volvox-like green alga. The S. robusta chloroplast genome (150,905 bp) is the largest diatom chloroplast genome characterised to date, mainly due to the presence of four large gene-poor regions. Open reading frames (ORFs) encoded by the gene-poor regions show similarity to putative proteins encoded by the chloroplast genomes of different heterokonts, as well as the plasmids pCf1 and pCf2 found in the diatom Cylindrotheca fusiformis. A tyrosine recombinase and a serine recombinase are encoded by the S. robusta chloroplast genome, indicating a possible mechanism for the introduction of novel genes. A plasmid with similarity to pCf2 was also identified. Phylogenetic analyses of three ORFs identified on pCf2 suggest that two of them are part of an operon-like gene cluster conserved in bacteria. Several genetic elements have moved through horizontal gene transfer between the chloroplast genomes of different heterokonts. Two recombinases are likely to promote such gene insertion events, and the plasmid identified may act as vectors in this process. The copy number of the plasmid was similar to that of the plastid genome indicating a plastid localization.