How the green alga Chlamydomonas reinhardtii keeps time

Abstract

The unicellular green alga Chlamydomonas reinhardtii has two flagella and a primitive visual system, the eyespot apparatus, which allows the cell to phototax. About 40 years ago, it was shown that the circadian clock controls its phototactic movement. Since then, several circadian rhythms such as chemotaxis, cell division, UV sensitivity, adherence to glass, or starch metabolism have been characterized. The availability of its entire genome sequence along with homology studies and the analysis of several sub-proteomes render C. reinhardtii as an excellent eukaryotic model organism to study its circadian clock at different levels of organization. Previous studies point to several potential photoreceptors that may be involved in forwarding light information to entrain its clock. However, experimental data are still missing toward this end. In the past years, several components have been functionally characterized that are likely to be part of the oscillatory machinery of C. reinhardtii since alterations in their expression levels or insertional mutagenesis of the genes resulted in defects in phase, period, or amplitude of at least two independent measured rhythms. These include several RHYTHM OF CHLOROPLAST (ROC) proteins, a CONSTANS protein (CrCO) that is involved in parallel in photoperiodic control, as well as the two subunits of the circadian RNA-binding protein CHLAMY1. The latter is also tightly connected to circadian output processes. Several candidates including a significant number of ROCs, CrCO, and CASEIN KINASE1 whose alterations of expression affect the circadian clock have in parallel severe effects on the release of daughter cells, flagellar formation, and/or movement, indicating that these processes are interconnected in C. reinhardtii. The challenging task for the future will be to get insights into the clock network and to find out how the clock-related factors are functionally connected. In this respect, system biology approaches will certainly contribute in the future to improve our understanding of the C. reinhardtii clock machinery.

CONSTANS and the evolutionary origin of photoperiodic timing of flowering

“…The influence of post-translational modifications in clock proteins is a characteristic that is gaining more and more importance in the concept of circadian clocks (Mizoguchi et al, 2006;Mehra et al, 2009). The influence of the clock in the photoperiod response and other crucial developmental processes of plants (Mas and Yanovsky, 2009;Imaizumi, 2010) and algae (Schulze et al, 2010) has recently been reviewed. In this review some of the aspects that connect photoperiod and circadian regulation, common features that seem to have arisen very early in the lineage of the photosynthetic eukaryotes (Matsuo et al, 2008), will be briefly discussed.…”

A Flavin Binding Cryptochrome Photoreceptor Responds to Both Blue and Red Light in Chlamydomonas reinhardtii

“…In addition to the CRYs, several proteins have been characterized in the past few years that are either partially or closely connected to the central oscillator of C. reinhardtii; a knockout or knockdown of these genes may cause phase shifts, arrhythmicity, or period lengthening or shortening (Schulze et al, 2010;Matsuo and Ishiura, 2011). These clock-related proteins include the two subunits of the RNA binding protein CHLAMY1 (C1 and C3 subunits; Iliev et al, 2006), several Rhythm Of Chloroplast (ROC) proteins (Matsuo et al, 2008), Casein Kinase1 (CK1; Schmidt et al, 2006), and Constans (CO), which is also involved in photoperiodic signaling (Serrano et al, 2009).…”

Phase-resetting mechanism of the circadian clock in Chlamydomonas reinhardtii

“…Clock genes/proteins have been identified in several organisms, including mammals, insects, fungi, land plants, cyanobacteria, and, recently, green algae (1,2,(5)(6)(7)(8). Except for general kinases and phosphatases (9), most of the components of circadian clocks are not conserved among evolutionarily divergent organisms.…”