The role of FraI in cell-cell communication and differentiation in the hormogonia-forming cyanobacterium Nostoc punctiforme

Multicellular cyanobacteria, like Nostoc punctiforme , rely on septal junctions for cell-cell communication, which is crucial for coordinating various physiological processes including differentiation of N ₂ -fixing heterocysts, spore-like akinetes and hormogonia - short, motile filaments important for dispersal. In this study we functionally characterize a protein, encoded by gene NpF4142 , which in a random mutagenesis approach, initially showed a motility-related function. The reconstructed NpF4142 knockout mutant exhibits further distinct phenotypic traits, including altered hormogonia formation with significant reduced motility, inability to differentiate heterocysts and filament fragmentation. For that reason, we named the protein FraI (fragmentation phenotype). The mutant displays severely impaired cell-cell communication, due to almost complete absence of the nanopore array in the septal cell wall, which is an essential part of the septal junctions. Despite lack of communication, hormogonia in the Δ fraI mutant maintain motility and phototactic behaviour, even though less pronounced than the wild type. This suggests an alternative mechanism for coordinated movement beyond septal junctions. Our study underscores the significance of FraI in nanopore formation and cell differentiation and provides additional evidence for the importance of septal junction formation and communication in various differentiation traits of cyanobacteria. The findings contribute to a deeper understanding of the regulatory networks governing multicellular cyanobacterial behaviour, with implications for broader insights into microbial multicellularity.