1. Department of Botany, National Museum of Nature and Science, Amakubo 4-1-1, Tsukuba, Ibaraki, 305-0005, Japan.
2. Center for Molecular Biodiversity Research, National Museum of Nature and Science, Amakubo 4-1-1, Tsukuba, Ibaraki, 305-0005, Japan.
3. Biodiversity Division, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan.
The complete genome of Annamia dubia was sequenced. The genome size is 4.02 Mbp, including 3886286 bp circular chromosome and four circular plasmids (31516, 42453, 38085 and 24903 bp). It included 3718 protein-coding sequences, 45 tRNA genes, three sets of rRNA genes, a microcystin biosynthesis gene cluster and six CRISPR (clustered regularly interspaced short palindromic repeat). Annamia is the only one genus in the Chroococcales that makes filamentous colonies. FraC and FraG were identified in the genome. These genes are required for the integrity of cell junctions and influencing filament integrity and are thought to be related to colony formation. These genes are first reported from Chroococcales, and may play a significant role in the colony formation of this species.
In the phylogenetic tree of the FraC gene, A. dubia was located in the basal position of Oscillatoriales. The GC ratio of FraC gene of A. dubia is very low from the genome and the FraC gene of Microcoleaceae. The presence of these genes in the basal region and the low GC ratio suggests that the FraC gene in this species was introduced by horizontal gene transfer. Since the filamentous colony is a fundamental and important taxonomic feature for the classification of cyanobacteria, the possibility of horizontal transmission of genes involved in filamentous cyanobacterial colonies is an important discovery for the classification of cyanobacteria.