foliaceum and H. akashiwo. All residues that are critical for catalytic activity of tyrosine recombinases are conserved in the S. robusta TyrC, similar to its heterokont homologues ( Fig. A.6A). Phylogenetic analyses ( Fig. A.6B) showed that heterokont (and dinoflagellate) TyrC forms a clade together with the Int recombinase encoded by the chloroplast genome of the green alga Oegodonium cardiacum ( Brouard et al., 2008). Another eukaryotic clade is formed by recombinases encoded by check details the mitochondrial genome of two other green algae, Prototheca wickerhamii ( Wolff et al., 1994) and Chaetosphaeridium globosum ( Turmel et al., 2002a). XerCD family tyrosine recombinases with a lower similarity

to TyrC are found in a large number of bacteria, mainly belonging to Firmicutes. A bacteria belonging to this phylum may be the source of the ancestral lateral gene FG-4592 supplier transfer of a tyrosine recombinase to an algal organellar genome. Expression analyses indicated that neither tyrC nor serC2 were expressed ( Fig. 6). Based on the presence of serine recombinases in the pCf1 and pCf2

plasmids (Hildebrand et al., 1992), SerC2 in the S. robusta chloroplast genome has likely also been associated with a plasmid, possibly a predecessor of pSr1. After integration of pSr1 in the chloroplast genome, the serC2 gene may have been lost from the plasmid. One second possible role for TyrC could be to act in conversion of multimeric chloroplast molecules to monomers, as has been speculated for the H. akashiwo TyrC ( Cattolico et al., 2008). A XerCD family recombinase has been shown to mediate excision of a genomic island from the genome of the bacterial pathogen Helicobacter pylori;

conjugative transfer of such genomic islands is believed to contribute to the genetic diversity of H. pylori ( Fischer et al., 2010). Whether a similar role can be attributed to TyrC in the chloroplast genomes of S. robusta and other eukaryotes warrants further experimentation. The occurrence of gene-poor regions containing uncharacterised ORFs appears to coincide with the presence of a serine recombinase (Fistulifera sp.), a tyrosine recombinase (H. akashiwo), or both (S. robusta and K. foliaceum) in the chloroplast genome ( Table 2). The chloroplast genomes of P. tricornutum, T. pseudonana and the diatom endosymbiont of D. baltica do not encode any recombinase; none of the ORFs listed in Table 2 are found in these diatoms, and the mean intergenic spacer is smaller ( Table 1). Interestingly, an ORF encoding a partial serine recombinase (annotated as Escp117) is found in the chloroplast genome of the brown alga Ectocarpus siliculosus ( Le Corguillé et al., 2009). The intergenic regions of the E. siliculosus chloroplast genome are longer than those of another brown alga, F. vesiculosus, where no traces of any recombinase were found.