Biology subjects

Risser, D.

Publications and source records attributed to Risser, D..

3 recordsLinked to original sources

bioRxiv2024

Cyanoexosortase B is essential for motility, biofilm formation and scytonemin production in a filamentous cyanobacterium

Exosortases are involved in trafficking proteins containing PEP-CTERM domains to the exterior of gram-negative bacterial cells. The role of these proteins in cyanobacteria, where such homologs are common, has not been defined. The filamentous cyanobacterium Nostoc punctiforme contains a single putative exosortase, designated cyanoexosortase B (CrtB), implicated by previous work in both the motility, and in the production of the UV-absorbing pigment, scytonemin. To determine the role of crtB in N. punctiforme, a crtB-deletion strain ({Delta}crtB) was generated. {Delta}crtB presented the loss of motility, biofilm formation, and scytonemin production. In the case of motility, the {Delta}crtB mutant exhibited a specific defect in the ability of hormogonia (specialized motile filaments) to adhere to hormogonium polysaccharide (HPS) and several PEP-CTERM proteins expressed in motile hormogonia were differentially abundant in the exoproteome of the wild type compared to the {Delta}crtB strain. These results are consistent with the hypothetical role of CrtB in the processing and export of PEP-CTERM proteins that play a critical role in stabilizing the interaction between the filament surface and HPS to facilitate motility and biofilm formation. In the case of scytonemin, the late biosynthetic steps of which occur in the periplasm and whose operon contains several putative PEP-CTERM proteins, {Delta}crtB failed to produce it. Given the abundance of putative PEP-CTERM proteins encoded in the N. punctiforme genome, and the fact that this study only associates a fraction of them with biological functions, it seems likely that CrtB may play an important role in other biological processes in cyanobacteria. ImportanceIn gram-negative bacteria, exosortases facilitate the trafficking of proteins to the exterior of the cell where they have been implicated in stabilizing the association of extracellular polymeric substances (EPS) with the cell surface to facilitate biofilm formation and flocculation, but the role of exosortases in cyanobacteria has not been explored. Here, we characterize the role of cyanoexosortase B (CrtB) in the filamentous cyanobacterium Nostoc punctiforme, demonstrating that crtB is essential for motility, biofilm formation, and the production of the sunscreen pigment scytonemin. These findings have important implications for understanding motility and biofilm formation in filamentous cyanobacteria as well as efforts toward heterologous production of scytonemin in non-native hosts.

microbiology↗

bioRxiv2024

Diel expression dynamics in filamentous cyanobacteria

Filamentous cyanobacteria of the Nostocaceae family are able to differentiate into multicellular forms to adapt to environmental stresses, and members can establish symbiosis with various embryophytes. Representative laboratory strains are typically grown under continuous light to maintain stable metabolic conditions, however, this departure from a natural diel cycle can result in extended stress. Early genomic examination of Nostoc punctiforme suggests the genetic potential for a circadian clock, but we lack insight into global cellular dynamics through the natural diel cycle for this model organism. Here, we comprehensively assess changes in expression of core cellular processes and the mobilome of accessory genetic elements during diel growth of N. punctiforme PCC 73102. The primary transcriptome confirmed that multicellular cyanobacteria precisely coordinate photosynthesis and carbon assimilation for cell division during the day, while control of DNA recombination and repair appeared to be sequestered to darkness. Moreover, we expanded the known repertoire of light sensing proteins to uncover a putative regulator of circadian rhythm that itself exhibits striking oscillation between day-night expression. This was in sharp contrast to the arrhythmic pattern observed for a homolog of the canonical circadian regulator in unicellular cyanobacteria. Looking beyond cellular coordination of diel growth, we uncovered dynamic mobile elements, and notably, targeted hypermutation by retroelements that are likely maintained for conflict mitigation, which is crucial to a multicellular lifestyle.

microbiology↗

bioRxiv2024

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 N2-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 {Delta}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. ImportanceThe filament-forming cyanobacterium Nostoc punctiforme serves as a valuable model for studying cell differentiation, including the formation of nitrogen-fixing heterocysts and hormogonia. Hormogonia filaments play a crucial role in dispersal and plant colonization, providing a nitrogen source through atmospheric nitrogen fixation, thus holding promise for fertilizer-free agriculture. The coordination among the hormogonia cells enabling uniform movement towards the positive signal remains poorly understood. This study investigates the role of septal junction-mediated communication in hormogonia differentiation and motility, by studying a {Delta}fraI mutant with significantly impaired communication. Surprisingly, impaired communication does not abolish synchronized filament movement, suggesting an alternative coordination mechanism. These findings deepen our understanding of cyanobacterial biology and have broader implications for multicellular behavior in prokaryotes.

microbiology↗