Biochemistry That Times the Day

“Timing is everything” appears to be a motto not limited to human existence. In fact, keeping track of time is so important that organisms ranging from algae and insects to mammals rely on circadian clocks, endogenous molecular oscillators with a period of approximately 24 h, to regulate diverse processes that include the cell cycle, gene expression, and metabolism. Although all biological clocks are characterized by three diagnostic properties, (i) an internal timer ticking with an ∼24 h period, (ii) entrainment of the endogenous timer by exogenous cues such as light, and (iii) temperature compensation, the molecular players constituting the clock are not conserved across all organisms. Beyond similarities and differences as far as the cogs and gears that assemble the clock are concerned, recent research has brought to light the increasingly complex nature of clock mechanism and regulation. Thus, the assumption that a so-called transcription/translationbased feedback loop (TTFL) is at the heart of all biological clocks appears to be an oversimplification at best; in all systems, a post-translational component of the timing circuitry is becoming apparent. This issue of Biochemistry bears witness to the advances in clock research achieved over the past decade by presenting a collection of seven Current Topics articles (DOIs 10.1021/bi5007354, 10.1021/bi500707c, 10.1021/ bi500731f, 10.1021/bi5005624, 10.1021/bi500922q, 10.1021/ bi501089x, and 10.1021/bi5008386). These papers penned by a diverse group of clock researchers attest to the universal importance of circadian timing and provide insight into progress in regard to clock structure, function, and mechanism in all walks of life, from seasonal flowering to circadian rhythmicity in carcinogenesis and chemotherapy.