Anabaena circadian clock behavior under nitrogen-poor conditions from single-cell measurements of fluorescence intensity

Circadian clock arrays in multicellular filaments of the
heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 display
remarkable spatio-temporal coherence under nitrogen-replete conditions. To
shed light on the interplay between circadian clocks and the formation of
developmental patterns, we followed the expression of a clock-controlled
gene under nitrogen deprivation, at the level of individual cells. Our
experiments showed that differentiation into heterocysts took place
preferentially within a limited interval of the circadian clock cycle,
that gene expression in different vegetative intervals along a developed
filament was discoordinated, and that the circadian clock was active in
individual heterocysts. Furthermore, Anabaena mutants lacking the kaiABC
genes encoding the circadian clock core components produced heterocysts
but failed in diazotrophy. Therefore, genes related to some aspect of
nitrogen fixation, rather than early or mid-heterocyst differentiation
genes, are likely affected by the absence of the clock. A bioinformatics
analysis supports the notion that RpaA may play a role as master regulator
of clock outputs in Anabaena, the temporal control of differentiation by
the circadian clock and the involvement of the clock in proper
diazotrophic growth. Together, these results suggest that under
nitrogen-deficient conditions, the clock coherent unit in Anabaena is
reduced from a full filament under nitrogen-rich conditions to the
vegetative cell interval between heterocysts.