The role of the protective shield against UV-C radiation and its molecular interactions in Nostoc species (Cyanobacteria)

Abstract

Cyanobacteria possess special defense mechanisms to protect themselves against ultraviolet (UV) radiation. This study combines experimental and computational methods to identify the role of protective strategies in Nostoc species against UV-C radiation. To achieve this goal, various species of the genus Nostoc from diverse natural habitats in Iran were exposed to artificial UV-C radiation. The results indicated that UV-C treatment significantly reduced the photosynthetic pigments while simultaneously increasing the activity of antioxidant enzymes. Notably, N. sphaericum ISB97 and Nostoc sp. ISB99, the brown Nostoc species isolated from habitats with high solar radiations, exhibited greater resistance compared to the green-colored species. Additionally, an increase in scytonemin content occurred with a high expression of key genes associated with its synthesis (scyF and scyD) during the later stages of UV-C exposure in these species. The molecular docking of scytonemin with lipopolysaccharides of the cyanobacteria that mainly cover the extracellular matrix revealed the top/side positioning of scytonemin on the glycans of these lipopolysaccharides to form a UV-protective shield. These findings pave the way for exploring the molecular effects of scytonemin in forming the UV protection shield in cyanobacteria, an aspect that has been ambiguous until now.

Keywords: Nostoc; Extracellular polysaccharide; Lipopolysaccharides; Molecular docking; Scytonemin; UV radiation.

Figure 1

Effect of UV-C radiation on the EPS contents. Values are presented as mean ± standard deviation. Results are expressed as means of three replicates.

Figure 2

SEM images of (a) the extracted RPS from N. carneum ISB88, (b) the extracted CPS from N. commune ISB98, (c) N. commune ISB98 (the arrow indicates the location of CPS), (d) N. carneum ISB88, (e) N. sphaericum ISB97, and (f) Nostoc sp. ISB99 filaments. Scale bar = 20 µm.

Figure 3

Effect of UV-C on the contents of chlorophyll-a (a), carotenoids (b), Scytonemin (c), and MAAs (d) in Nostoc commune ISB98, Nostoc carneum ISB88, Nostoc sphaericum ISB97, and Nostoc sp. ISB99. Results are expressed as the means of three replicates. Vertical bars indicate standard deviation of the means.

Figure 4

Effect of UV-C radiation on (a) Catalase (CAT) and (b) Superoxide Dismutase (SOD) activities. SOD and CAT activities were measured at different time intervals and compared to the enzyme activitiy at time 0 (control). Measurements were performed in triplicate, and vertical bars indicate the standard deviation of the means.

Figure 5

qRT-PCR expression analysis of genes involved in scytonemin biosynthesis (scyF, scyE , scyD) (a, b, c), MAAs biosynthesis (mysD) (d), and capsular polysaccharides biosynthesis (Nos7107 and Nos7524) (e, f) in N. commune ISB98, N. carneum ISB88, N. sphaericum ISB97, and Nostoc sp. ISB99. Results are expressed as the means of three replicates.

Figure 6

Binding poses of scytonemin-LPS complex. The blue atom name spheres indicate the lipid A section of LPS complex. The green atom name spheres indicate the core oligosaccharide section of LPS complex and the yellow atom name spheres show the scytonemin molecule.

Figure 7

Distribution of binding pose clusters based on ligand RMSD = 2.0

Figure 8

3D structure of scytonemin binding to LPS complex with hydrogen bond interactions. The blue atom name sticks indicates the lipid A section of the LPS complex. The green atom name sticks indicate the core oligosaccharide section of the LPS complex and the yellow atom name sticks show the scytonemin molecule. The purple dashed line one shows the H-bonds interactions in the LPS complex and the white color indicates the hydrogen atom.