Biased inheritance of the protein PatN frees vegetative cells to initiate patterned heterocyst differentiation

Abstract

Heterocysts, cells specialized for nitrogen fixation in certain filamentous cyanobacteria, appear singly in a nonrandom spacing pattern along the chain of vegetative cells. A two-stage, biased initiation and competitive resolution model has been proposed to explain the establishment of this spacing pattern. There is substantial evidence that competitive resolution of a subset of cells initiating differentiation occurs by interactions between a selfenhancing activator protein, HetR, and a diffusible pentapeptide inhibitor PatS-5 (RGSGR). Results presented here show that the absence of a unique membrane protein, PatN, in Nostoc punctiforme strain ATCC 29133 leads to a threefold increase in heterocyst frequency and a fourfold decrease in the vegetative cell interval between heterocysts. A PatN-GFP translational fusion shows a pattern of biased inheritance in daughter vegetative cells of ammonium-grown cultures. Inactivation of another heterocyst patterning gene, patA, is epistatic to inactivation of patN, and transcription of patA increases in a patN-deletion strain, implying that patN may function by modulating levels of patA. The presence of PatN is hypothesized to decrease the competency of a vegetative cell to initiate heterocyst differentiation, and the cellular concentration of PatN is dependent on cell division that results in cells transiently depleted of PatN. We suggest that biased inheritance of cell-fate determinants is a phylogenetic domain-spanning paradigm in the development of biological patterns.activator-inhibitor | pattern differentiation T wo broad strategies used in the development of biological patterns are activator-inhibitor systems, a refinement of Turing-type reaction-diffusion systems (1, 2), and differential inheritance of cell fate determinants (3, 4). These strategies have been analyzed in multicellular eukaryotes ranging in developmental complexity from hydra to vertebrates (1, 4). Some filamentous cyanobacteria show spaced patterns of differentiated cells, such as nitrogen-fixing heterocysts and spore-like akinetes, thereby representing the simplest and oldest evolutionary model for study of the development of biological patterns and multicellular interactions. There is strong experimental evidence that an activatorinhibitor system regulates heterocyst patterning in filamentous cyanobacteria (5-7). We report here the characteristics of a gene (patN) that is required for normal heterocyst patterning, and the phenotype and protein localization of which is consistent with a role in the biased initiation of heterocyst differentiation via biased inheritance.Heterocysts terminally differentiate from vegetative cells in response to limitation of combined nitrogen. The heterocysts appear in a nonrandom spacing of approximately 1 heterocyst to every 10-15 vegetative cells. The activator-inhibitor system governing this patterning consists of a self-enhancing activator protein, HetR (8, 9), and its antagonist, proteins containing a pentapeptide, PatS-5 (RGSGR) (10, 11). These two primary positi...

Restricted cellular differentiation in cyanobacterial filaments

“…However, can any cell in a filament differentiate into a heterocyst? In PNAS, Risser et al (5) present evidence consistent with the idea that, at a given time, only some cells in the filament have the capability to differentiate.…”

“…In the patN mutant, vegetative cell intervals between heterocysts are shorter than in the wild type, but the distribution of heterocysts in the filaments is still nonrandom. Thus, the PatS-and HetNdependent lateral inhibition seems to operate in the patN mutant, and inhibition of differentiation by PatS has indeed been corroborated (5). Analyzing the fluorescence from a PatN-GFP fusion protein, Risser et al observe the protein in the periphery of the cell, consistent with a cytoplasmic membrane location, but with an uneven distribution between cells.…”

“…By performing genetic analysis, Risser et al (5) A two-stage model of heterocyst differentiation involving biased initiation followed by competitive resolution has been previously proposed (3). Whereas experimental support for competitive resolution of cells simultaneously starting differentiation was available (10-13), Risser et al (5) now identify a factor putatively influencing biased initiation of heterocyst differentiation. Genes encoding homologues to PatN are present in all heterocystforming cyanobacteria for which the complete genomic sequence is available, indicating that PatN may generally participate in heterocyst differentiation.…”

“…Mutants of this gene form heterocysts preferentially at the filaments termini (16), and PatA is postulated to counteract the negative effects of PatSand HetN-related regulators on heterocyst differentiation (2,4). By performing genetic analysis, Risser et al (5) A two-stage model of heterocyst differentiation involving biased initiation followed by competitive resolution has been previously proposed (3). Whereas experimental support for competitive resolution of cells simultaneously starting differentiation was available (10-13), Risser et al (5) now identify a factor putatively influencing biased initiation of heterocyst differentiation.…”

“…Second, how does PatN impede cell differentiation? First, analyzing a transcriptional fusion of the gfp gene to the patN gene promoter, Risser et al (5) show that the expression of patN takes place in all cells of the filament when they grow in the presence of ammonium, as well as after nitrogen deprivation. This implies that the uneven distribution of PatN in the filaments results from posttranslational regulation rather than from differential gene expression.…”