Comparative genomics of Dolichospermum circinale strains with differential paralytic shellfish toxin profiles

The cyanobacterium Dolichospermum circinale is a known producer of the neurotoxin saxitoxin and its analogues, collectively known as the paralytic shellfish toxins (PSTs). PSTs vary in potency, and the reported toxin profiles of D. circinale blooms vary in the quantities of individual PSTs, with the regulation of these profiles being poorly understood. In this study, we present the genomes of four D. circinale strains (ACBU01, ACMB03, ACMB13 and FSS-124) with unique PST profiles and perform genome-wide comparisons and specific analysis of the PST-producing biosynthetic gene cluster (sxt) to understand the variability in PST quotas. A reassessment of the previously published D. circinale AWQC131C genome was also performed to collate genomic variation between all strains. Analysis at the nucleotide and amino acid sequence level revealed that toxic strains maintain high genome-wide similarities, corroborated by the analysis of the pan- and variable genomes of each strain. Specifically, the sxt gene sequences were 99-100% identical across all strains. Novel tailoring (sxtSUL, sxtDIOX) and transport (sxtM4) genes were identified within the sxt cluster that were not reported previously in D. circinale. Taken together, these results indicate that the genetic machinery involved in PST production is conserved in this species, suggesting that the regulation of PST biosynthesis in D. circinale does not occur at the genomic level.