ABSTRACT Circadian clock arrays in multicellular filaments 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 differentiation differentiation into heterocysts took place preferentially within a limited interval of the circadian clock cycle, that gene expression in different different different vegetative intervals along a developed filament 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 differentiation differentiation genes, are likely affected affectedaffected 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 differentiation differentiation by the circadian clock and the involvement of the clock in proper diazotrophic growth. Together, these results suggest that under nitrogen-defi nitrogen-defi nitrogen-defi cient cient conditions, the clock coherent unit in Anabaena is reduced from a full filament filamentfilamentunder nitrogen-rich conditions to the vegetative cell interval between heterocysts.

The work in Rehovot was supported by the Minerva Stiftung to J.S. J.S. is the incumbent of a Siegried and Irma Ullman Professorial Chair. The work in Seville was supported by grant no. PID2020-118595GB-100 from Agencia Estatal de Investigación, Spain, and the European Regional Development Fund to A.H. and E.F.

Topic name Circadian Rhythm; Cyanobacteria; Bacterial Protein Prominence percentile 71.953

Chemicals and CAS Registry Numbers nitrogen 7727-37-9 Nitrogen

Peer reviewed