Project Details
Projekt
Determinants of Heterocyst Differentiation in Multicellular Cyanobacteria
Subject Area
Metabolism, Biochemistry and Genetics of Microorganisms
Term
since 2022
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 503940974
Nostocales cyanobacteria develop trichomes with cells communicating through gated septal junctions and responding to certain stimuli as a whole. They differentiate several cell types, including heterocysts specialized for the fixation of dinitrogen gas (N2) using the enzyme nitrogenase. Thus, these cyanobacteria fulfill all hallmarks of true multicellular organisms. Heterocysts and neighboring photosynthetically active vegetative cells crucially depend on each other and exchange metabolites and signals, which makes the heterocyst-specific N2 fixation the emergent function that allows to survive in nitrogen-poor environments. Upon removal of combined nitrogen from the environment, mature heterocysts differentiate from vegetative cells over ~24 h, making the process accessible for detailed research. Our confocal microscopy studies with suitable promoter-reporter gene fusions and flow cytometry analyses of single cells showed that the vegetative cells in a filament in the presence of combined nitrogen are not uniform, but exist in different states. These pre-existing heterogeneities may make some cells more suitable to enter the heterocyst pathway than others (hypothesis 1). The different cell states may result from the stochastic nature of transcription, i.e. transcriptional bursts transiently activating hetR transcription (hypothesis 2), which is studied by single-cell RNA-Seq. Previous approaches often screened for the inability to grow on N2, in which factors affecting patterning or time of commitment as well as essential and redundant genes would have been missed. Therefore, our hypothesis 3 posits that there are factors that have not been previously characterized or recognized for their role in heterocyst differentiation. Indeed, we identified several RNA-binding proteins and sRNAs that are of critical relevance for differentiation and pattern formation. Our preliminary results furthermore point at the role of a match-making RNA chaperone in this process and the involvement of ribosome heterogeneity. Based on the peptide character of morphogenic signals, we assume further that unrecognized low-molecular-weight factors exist (hypothesis 4) and we will search for such factors using MALDI-MS. However, overall, we will shift the emphasis of the project more toward the characterization of the newly identified factors, which will be crucial for our work on hypothesis 5, that there is a well-structured mechanism that coordinates the cell-type-specific proteome composition, with sRNAs, RNA chaperones, and some type of specialized ribosomes as critical components. For decades, heterocyst differentiation was thought to be mainly controlled by transcription factors and corresponding transcriptome changes. Our results may lead to a paradigm shift that the transcriptional regulation is complemented by an equally important post-transcriptional mechanism controlling the proteome.
DFG Programme
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