1), which in total contributes about 24,000 bp (16%) to Inhibitor Library cell assay the genome size. Another consequence of the gene-poor regions is a large average size of the intergenic spacers (214.0 bp). Excluding these regions, the average intergenic spacer size is reduced to 134.8 bp. An interesting

feature of the S. robusta chloroplast genome is the presence of introns in two of the genes: the rnl gene encoding the 23S ribosomal RNA in the IR and the atpB gene encoding the ATP synthase beta chain. The other diatom chloroplast genomes analysed so far do not contain any introns. The only intron reported in a heterokont chloroplast genome so far is a group I intron found in the trnL gene of Fucus vesiculosus and a few other brown algae ( Le Corguillé et al., 2009). The S. robusta rnl gene contains a group I intron with a length

of 764 bp that falls within the subgroup IA3 ( Michel et al., 1990). This type of introns has self-splicing activity, and is mostly found in fungi, plants and red and green algae ( Haugen et al., 2005). The rnl intron contains an ORF encoding a putative endonuclease with a single LAGLIDADG domain. Single-LAGLIDADG endonucleases form homodimers that recognise Akt inhibitor and cleave palindromic or pseudopalindromic DNA target sites ( Chan et al., 2011). Phylogenetic analyses ( Fig. 2A) indicated that the S. robusta endonuclease ORF (designated I-SroI according to standard nomenclature for the family ( Belfort

and Roberts, 1997)) is similar to single-LAGLIDADG endonucleases from green algae (chlorophytes) ( Heath et al., 1997 and Lucas et al., 2001), streptophytes ( Turmel et al., 2002b) and the amoeboid protozoan Acanthamoeba castellanii ( Lonergan and Gray, 1994). All residues that are conserved within LAGLIDADG endonucleases in green algae are also conserved in I-SroI, with the exception of Asp93 in I-SroI, which is a highly conserved proline in the other members of the family ( Fig. A.1) ( Lucas et al., 2001). The conserved proline is part of the hydrophobic core of LAGLIDADG endonucleases ( Heath FAD et al., 1997); replacing it with an acidic residue may therefore have deleterious effects on the structure and activity. Homing endonucleases, such as LAGLIDADG endonucleases that reside within self-splicing introns, have evolved to act as opportunistic selfish DNA considered to provide little benefit to their hosts ( Stoddard and Belfort, 2010). However, homing endonucleases may also drive important gene conversion events. The HO endonuclease in Saccharomyces cerevisiae, which is of the LAGLIDADG type, is responsible for mating-type genetic switch ( Jin et al., 1997). Further evidence for a green algal ancestry of the S. robusta rnl intron was found in the non-coding part of the intron.