LdpA: a component of the circadian clock senses redox state of the cell

Abstract

The endogenous 24-h (circadian) rhythms exhibited by the cyanobacterium Synechococcus elongatus PCC 7942 and other organisms are entrained by a variety of environmental factors. In cyanobacteria, the mechanism that transduces environmental input signals to the central oscillator of the clock is not known. An earlier study identified ldpA as a gene involved in light-dependent modulation of the circadian period, and a candidate member of a clock-entraining input pathway. Here, we report that the LdpA protein is sensitive to the redox state of the cell and exhibits electron paramagnetic resonance spectra consistent with the presence of two Fe4S4 clusters. Moreover, LdpA copurifies with proteins previously shown to be integral parts of the circadian mechanism. We also demonstrate that LdpA affects both the absolute level and light-dependent variation in abundance of CikA, a key input pathway component. The data suggest a novel input pathway to the circadian oscillator in which LdpA is a component of the clock protein complex that senses the redox state of a cell

Torn apart and reunited: impact of a heterotroph on the transcriptome of Prochlorococcus

“…For example, transcripts encoding LdpA, an input sensor to the circadian clock (Ivleva et al, 2005), were significantly less abundant following 24 h of coculture, and transcripts for KaiB, one of the central circadian clock proteins, were less abundant after 48 h (Supplementary Table S2). Prochlorococcus has only a partial circadian oscillator that likely needs to be reset daily by sunlight, and so the function of this regulatory oscillator is thought to be influenced by the cellular redox state (Axmann et al, 2009).…”

The pseudo‐receiver domain of CikA regulates the cyanobacterial circadian input pathway

“…In the context of the cell, this core oscillation is integrated with other events that allow synchronization of the circadian oscillation (involving CikA) and relay of temporal information to clock-controlled processes. LdpA copurifies with at least KaiA and CikA, suggesting physical proximity in the cell (Ivleva et al, 2005). However, yeast 2-hybrid assays do not reveal a direct connection between CikA and any of the Kai proteins (S. R. Mackey et al, manuscript in preparation).…”

“…A cikA null strain still exhibits circadian rhythmicity as monitored by bioluminescence produced from luciferase reporter genes; however, the circadian period is shortened by about 2 h, the amplitude of oscillation is greatly reduced, and, diagnostic of an input pathway defect, the ability to sense a 5 h pulse of darkness that resets the phase of circadian rhythms in the wild-type (WT) strain is almost completely abolished (Schmitz et al ., 2000). Other factors that affect environmental sensing by the clock have been identified (Kutsuna et al ., 1998;Ivleva et al ., 2005), but none has as great an effect as CikA on the ability to reset the clock other than a component of the circadian oscillator itself (Kiyohara et al ., 2005); the oscillator is presumably the target of the input pathway (Williams et al ., 2002).…”

“…Copurification of CikA and Kai proteins with LdpA, another input pathway component, suggests that the input pathway is physically associated with the periodosome (Ivleva et al ., 2005). LdpA affects both light-dependent abundance of CikA and the sensitivity of CikA to cellular redox state.…”

“…The requirement of LdpA for modulation of CikA abundance according to light intensity may be sufficient to explain the insensitivity of circadian period to light intensity that is observed in an ldpA null mutant. A yeast two-hybrid assay does not detect direct interaction between CikA and LdpA, or between either of them and any individual Kai protein (Ivleva et al ., 2005). Thus, it is expected that intermediates are involved in the transfer of information from CikA to the Kai oscillator (Golden, 2003).…”