DesC1 and DesC2, Δ9 Fatty Acid Desaturases of Filamentous Cyanobacteria: Essentiality and Complementarity.

DesC1 and DesC2, which are fatty acid desaturases found in cyanobacteria, are responsible for introducing a double bond at the Δ9 position of fatty-acyl chains, which are subsequently esterified to the sn-1 and sn-2 positions of the glycerol moiety, respectively. However, since the discovery of these two desaturases in the Antarctic cyanobacterium Nostoc sp. SO-36, no further research has been reported. This study presents a comprehensive characterization of DesC1 and DesC2 through targeted mutagenesis and transformation using two cyanobacteria strains: Anabaena sp. PCC 7120, comprising both desaturases, and Synechocystis sp. PCC 6803, containing a single Δ9 desaturase (hereafter referred to as DesCs) sharing similarity with DesC1 in amino acid sequence. The results suggested that both DesC1 and DesC2 were essential in Anabaena sp. PCC 7120 and that DesC1, but not DesC2, complemented DesCs in Synechocystis sp. PCC 6803. In addition, DesC2 from Anabaena sp. PCC 7120 desaturated fatty acids esterified to the sn-2 position of the glycerol moiety in Synechocystis sp. PCC 6803.

Possible involvement of extracellular polymeric substrates of Antarctic cyanobacterium Nostoc sp. strain SO-36 in adaptation to harsh environments.

Cyanobacteria are some of the primary producers in extremely cold biospheres such as the Arctic, Antarctic, and vast ice sheets. Many genera of cyanobacteria are identified from these harsh environments, but their specific mechanisms for cold adaptation are not fully understood. Nostoc sp. strain SO-36 is a cyanobacterium isolated in Antarctica more than 30 years ago and regarded as a psychrotolelant species. To determine whether the strain is psychrotolelant or psychrophilic, it was first grown at 30 °C and 10 °C. The cells grew exponentially at 30 °C, but their growth stopped at 10 °C, indicating that the strain is only psychrotolerant. Microscopic analysis revealed that the morphology of the cells grown at 30 °C was filamentous and differentiated heterocysts, which are specialized cells for gaseous nitrogen fixation under nitrogen-deprived conditions, indicating that the strain can grow diazotrophically. The cells grown at 10 °C have a smaller size, shortened filament length and decreased chlorophyll content per cell. At 10 °C, the cells are aggregated with extracellular polymeric substrates (EPSs), which is a common mechanism to protect cells from ultraviolet light. These results imply that segmentation into short filaments was induced by photodamage at low temperatures. To fully understand the adaptation mechanisms of Nostoc sp. strain SO-36 for low-temperature conditions, next-generation sequencing analyses were conducted. Complete genome sequence of the strain revealed that it has one main chromosome of approximately 6.8 Mbp with 4 plasmids, including 6855 coding sequences, 48 tRNA genes, 4 copies of rRNA operons, and 5 CRISPR regions. Putative genes for EPS biosynthesis were found to be conserved in Nostocaceae regardless of their habitat. These results provide basic information to understand the adaptation mechanisms at low temperatures, and the strain can be a model organism to analyze adaptation to extreme environments.