IntroductionThe basic helix-loop-helix PER-ARNT-SIM (bHLH-PAS) family of transcription factors respond to a wide range of external stimuli to regulate diverse biological processes ranging from development to circadian rhythms. These proteins selectively heterodimerize through relatively well-conserved bHLH and PAS domains, while differences in C-terminal regulatory domains confer opposing activities to either stimulate or repress transcription. The evolution of modular regulatory domains and the ability to selectively dimerize with different subunits within the family allows fine-tuning of gene expression through temporal and tissue-specific expression of bHLH-PAS subunits. The aim of this critical review is to discuss the functional specification through modular domain architecture of bHLH-PAS proteins. ConclusionThe modular domain architecture of bHLH-PAS transcription factors plays an integral role in their ability to keep mammals healthy and in tune with their environment.
Circadian clocks control gene expression to provide an internal representation of time. Here, we reconstituted in vitro under defined conditions the cyanobacterial circadian clock system which includes an oscillator, signal‐transduction pathways, transcription factor, and promoter DNA. The system oscillates autonomously, remains phase coherent for many days, and allows real‐time observation of each component simultaneously without user intervention. We show that the circadian sensor histidine kinase SasA engages directly with KaiB and KaiC proteins to regulate the period and enhance robustness of the reconstituted circadian oscillator in vitro, particularly under limiting concentrations of KaiB, by using structural mimicry to cooperatively recruit the rare, fold‐switched conformation of KaiB to the KaiC hexamer to form the nighttime repressive complex. The other circadian sensor histidine kinase, CikA, also contributes directly to robustness in vitro by enhancing rhythms under limiting concentrations of KaiA.
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