Abstract Nostoc punctiforme is a phenotypically com-
plex, filamentous, nitrogen-fixing cyanobacterium, whose
vegetative cells can mature in four developmental direc-
tions. The particular developmental direction is deter-
mined by environmental signals. The vegetative cell cycle
is maintained when nutrients are sufficient. Limitation for
combined nitrogen induces the terminal differentiation of
heterocysts, cells specialized for nitrogen fixation in an
oxic environment. A number of unique regulatory events
and genes have been identified and integrated into a work-
ing model of heterocyst differentiation. Phosphate limita-
tion induces the transient differentiation of akinetes,
spore-like cells resistant to cold and desiccation. A variety
of environmental changes, both positive and negative for
growth, induce the transient differentiation of hormogo-
nia, motile filaments that function in dispersal. Initiation
of the differentiation of heterocysts, akinetes and hor-
mogonia are hypothesized to depart from the vegetative
cell cycle, following separate and distinct events. N. punc-
tiforme also forms nitrogen-fixing symbiotic associations;
its plant partners influence the differentiation and behav-
ior of hormogonia and heterocysts. N. punctiforme is ge-
netically tractable and its genome sequence is nearly com-
plete. Thus, the regulatory circuits of three cellular differ-
entiation events and symbiotic interactions of N. puncti-
forme can be experimentally analyzed by functional ge-
nomics.
Abstract Nostoc punctiforme is a phenotypically com-
plex, filamentous, nitrogen-fixing cyanobacterium, whose
vegetative cells can mature in four developmental direc-
tions. The particular developmental direction is deter-
mined by environmental signals. The vegetative cell cycle
is maintained when nutrients are sufficient. Limitation for
combined nitrogen induces the terminal differentiation of
heterocysts, cells specialized for nitrogen fixation in an
oxic environment. A number of unique regulatory events
and genes have been identified and integrated into a work-
ing model of heterocyst differentiation. Phosphate limita-
tion induces the transient differentiation of akinetes,
spore-like cells resistant to cold and desiccation. A variety
of environmental changes, both positive and negative for
growth, induce the transient differentiation of hormogo-
nia, motile filaments that function in dispersal. Initiation
of the differentiation of heterocysts, akinetes and hor-
mogonia are hypothesized to depart from the vegetative
cell cycle, following separate and distinct events. N. punc-
tiforme also forms nitrogen-fixing symbiotic associations;
its plant partners influence the differentiation and behav-
ior of hormogonia and heterocysts. N. punctiforme is ge-
netically tractable and its genome sequence is nearly com-
plete. Thus, the regulatory circuits of three cellular differ-
entiation events and symbiotic interactions of N. puncti-
forme can be experimentally analyzed by functional ge-
nomics.
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