Paralytic shellfish poisoning (PSP) is a syndrome related to consumption of shellfish contaminated by toxigenic microalgae that biosynthesize the tetrahydropurine neurotoxin saxitoxin (STX) and/or various derivatives. Dinoflagellates are the main producers of PSP toxins in marine environments but PSP toxin production is also well described from freshwater cyanobacteria. Although the pathways involved in STX biosynthesis remain poorly known, studies with labeled precursors have suggested that dinoflagellates and cyanobacteria share this biosynthetic pathway. In contrast to the complexity of the dinoflagellate genome (3000215000 Mbp), the simplicity of cyanobacterial genomes (from 1.8 to 13.6 Mb) offers an alternative model for understanding the pathways involved in toxin production and gene regulation. The cyanobacterium Cylindrospermopsis raciborskii (genome size ca. 2.2 Mb) comprises strains able to produce either PSP toxins or the hepatotoxin cylindrospermopsin (CYN), as well as non-toxigenic strains. We have fully sequenced the complete genome of two strains of C. raciborskii that share 99.7% of their 16S rRNA sequences but differ in toxin profile (a CYN- versus a PSP toxin-producer). This project includes a study of: i) comparative genomics to identify the main differences between these strains that could be related to toxin production or regulation; and ii) physiological conditions that modify the toxin profile and its genetic regulation in the CYN- versus PSP toxin-producing strains using whole genome microarrays. The data for the cyanobacterium should be useful to evaluate homology of PSP toxin production among Cyanobacteria and Dinoflagellates and may assist in identifying the key genes responsible for PSP toxin synthesis in Dinoflagellates.