The plastid genome of the cryptophyte alga, Guillardia theta: Complete sequence and conserved synteny groups confirm its common ancestry with red algae

The plastid genome of the cryptophyte alga Guillardia theta (121,524 bp) ha s been completely sequenced. The genome is 33% G+C and contains a short, no nidentical inverted repeat (4.9 kb) encoding the two rRNA cistrons. The lar ge and small single-copy regions are 96.3 and 15.3 kb, respectively. Forty- six genes encoding proteins for photosynthesis, 5 genes for biosynthetic fu nction, 5 genes involved in replication and division, 30 tRNA genes, 44 rib osomal protein genes (26 large subunit and 18 small subunit), 3 translation factors, 8 genes encoding components of the transcriptional machinery incl uding 3 ycfs (hypothetical chloroplast frames), and 26 additional ycfs have been identified. There are eight ORFs larger than 50 amino acids, 3 of whi ch have homologues on the plastid genome of the rhodophyte, Porphyra purpur ea (Reith and Munholland 1995) and/or the Synechocystis genome (Kaneko et a l. 1996) and can be designated new ycfs. Intergenic spacers are very short, no introns have been detected, and several genes overlap, all resulting in a very compact genome. In addition, large clusters of genes (such as those for the ribosomal proteins) are organized into single transcriptional unit s (Wang et al. 1997), again resulting in an economically organized genome. The cryptophyte plastid genome is almost completely comprised of clusters o f genes that are found on the rhodophyte Porphyra purpurea, confirming its common ancestry with red algae. Furthermore, recombination events involving both tRNA genes and the rRNA cistrons appear to have been responsible for the structure of the cryptophyte plastid genome, including the formation of the inverted repeat.