Visible light alters yeast metabolic rhythms by inhibiting respiration

Abstract

Exposure of cells to visible light in nature or in fluorescence microscopy often is considered to be relatively innocuous. However, using the yeast respiratory oscillation (YRO) as a sensitive measurement of metabolism, we find that non-UV visible light has a significant impact on yeast metabolism. Blue/green wavelengths of visible light shorten the period and dampen the amplitude of the YRO, which is an ultradian rhythm of cell metabolism and transcription. The wavelengths of light that have the greatest effect coincide with the peak absorption regions of cytochromes. Moreover, treating yeast with the electron transport inhibitor sodium azide has similar effects on the YRO as visible light. Because impairment of respiration by light would change several state variables believed to play vital roles in the YRO (e.g., oxygen tension and ATP levels), we tested oxygen's role in YRO stability and found that externally induced oxygen depletion can reset the phase of the oscillation, demonstrating that respiratory capacity plays a role in the oscillation's period and phase. Light-induced damage to the cytochromes also produces reactive oxygen species that up-regulate the oxidative stress response gene TRX2 that is involved in pathways that enable sustained growth in bright visible light. Therefore, visible light can modulate cellular rhythmicity and metabolism through unexpectedly photosensitive pathways.M any organisms are exposed to visible light in their environment. Full sunlight can deliver up to 10 quadrillion photons of visible light·cm −2 ·s −1 (i.e., 2,000 μEinsteins·m −2 ·s −1 ), and cloud cover reduces this exposure only by a factor of 10. Even though sunlight provides photosynthetic energy to plants and a medium for the vision of many animal species, its highintensity rays damage living cells when light-absorbing molecules cannot safely disperse the energy that photons bring. Although the destructive capacity of UV light is widely appreciated, photons of visible light can be deleterious, e.g., by destroying cytochromes and thus affecting cellular respiration (1) or by producing reactive oxygen species (ROS) that cause damage to DNA, membranes, and other cellular components (2). To cope with the damaging effects of light, organisms have evolved different strategies ranging from the expression of shielding pigments, such as melanin and carotenoids (3), to active mechanisms that sense light and respond quickly to mitigate/repair damage, such as iris constriction to protect the retina (4, 5), light-avoidance movements by chloroplasts and mitochondria (6, 7), and the induction/activation of DNA photolyase (8). A third strategy to minimize damage from light is to anticipate and prepare for its effects through the use of cellular timing mechanisms such as a circadian clock (9).The budding yeast Saccharomyces cerevisiae lacks the wellcharacterized photoreceptors of many other organisms [e.g., cryptochromes, phytochromes, and the photoreceptors whitecollar 1 (WC-1), and rhodopsins] that sense light intensity and spe...

Dual-Color Monitoring Overcomes the Limitations of Single Bioluminescent Reporters in Fast-Growing Microbes and Reveals Phase-Dependent Protein Productivity during the Metabolic Rhythms of Saccharomyces cerevisiae

“…Alternatively, the Cry2 and Cib1 proteins from plants dimerize in the presence of blue light, which can in turn be used to activate a modified bacterial lexA promoter (28,34). And although we have previously shown that intense visible light perturbs the yeast oscillation, we have also shown that intensities less than 100 E/m 2 /s have little effect on the oscillation (9). A lightinducible promoter system offers the advantages of a heterologous gene control system that can be rapidly activated or inactivated without adding chemicals to the culture or disturbing the oscillation.…”

“…Rhythmic production of proteins from oscillating cultures can be more efficient than steady-state production, especially for unstable proteins and those strongly affected by cell cycle-dependent proteases (5,6). A respiratory oscillation manifests in yeast under specific conditions of continuous culture and exhibits 1-to 6-h rhythms of oxygen consumption, cell division, metabolite production, and gene expression (7)(8)(9)(10), all of which may make certain phases of the oscillation better for rhythmic protein production than others.…”

Chronobiology and circadian rhythms establish a connection to diagnosis

“…The growth of yeast strains that are null for the yeast activator protein-1 gene that regulates oxidative stress genes is exquisitely sensitive to visible light. This reveals that light can both modulate respiration and induce oxidative stress [7].…”

Metabolic Compensation and Circadian Resilience in Prokaryotic Cyanobacteria

“…Some researchers have proposed that high-frequency metabolic feedback loops—under selective pressure from the daily environment—were recruited to coalescence of an approximately daily period (16). Another model for the evolution of circadian timing—the so-called escape-from light hypothesis (7, 143, 144)—posits that light can damage cellular metabolism (e.g., DNA damage caused by UV light), which could have been a selective force for the evolution of circadian pacemakers. Briefly, the escape-from-light hypothesis proposes that light during the sunlit portion of the daily cycle had profound and mostly deleterious effects on early life; therefore, phasing cellular processes that are hypersensitive to light so that they occur in the dark nighttime may have driven the early evolution of circadian timers.…”

“…The authors of this paper suggested that most organisms that survived the transition to an aerobic environment 2.5 billion years ago were those that respired and/or evolved oxygen. Electron-transport chains involving oxygen inevitably produce ROS by-products, and due to daily cycles of photosynthesis and metabolism driven by daily LD and temperature changes, these ROS were produced rhythmically over the span of the daily cycle (113, 144). Therefore, a system to anticipate and manage the concomitant daily generation of intracellular ROS was adaptive.…”