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, 96 (6), 2261-7

A Mechanism of Energy Dissipation in Cyanobacteria

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A Mechanism of Energy Dissipation in Cyanobacteria

Rudi Berera et al. Biophys J.

Abstract

When grown under a variety of stress conditions, cyanobacteria express the isiA gene, which encodes the IsiA pigment-protein complex. Overexpression of the isiA gene under iron-depletion stress conditions leads to the formation of large IsiA aggregates, which display remarkably short fluorescence lifetimes and thus a strong capacity to dissipate energy. In this work we investigate the underlying molecular mechanism responsible for chlorophyll fluorescence quenching. Femtosecond transient absorption spectroscopy allowed us to follow the process of energy dissipation in real time. The light energy harvested by chlorophyll pigments migrated within the system and eventually reaches a quenching site where the energy is transferred to a carotenoid-excited state, which dissipates it by decaying to the ground state. We compare these findings with those obtained for the main light-harvesting complex in green plants (light-harvesting complex II) and artificial light-harvesting antennas, and conclude that all of these systems show the same mechanism of energy dissipation, i.e., one or more carotenoids act as energy dissipators by accepting energy via low-lying singlet-excited S(1) states and dissipating it as heat.

Figures

Figure 1
Figure 1
Absorption spectrum of IsiA aggregates at room temperature.
Figure 2
Figure 2
Kinetic traces in different regions of the spectrum along with the fit obtained from a global analysis with a sequential model. The amplitudes are in mOD; the inset shows the residuals.
Figure 3
Figure 3
(Upper) Kinetic model. (Lower) SADS obtained from the target analysis. (Green) Chl 1; (red) Chl 2, Chl 3, and Chl 4 (blue) Q.
Figure 4
Figure 4
Kinetic traces along with the fit obtained from the target analysis and the contributions of the various compartments. The green line represents the contribution of Chl 1; the red line represents the contribution of Chl 2, Chl 3, and Chl 4; and the blue line represents the contribution from the quencher. The amplitudes are in mOD.
Figure 5
Figure 5
Proposed quenching model. (Upper) Energy is transferred among the Chl pool and eventually reaches a quenching site where it is dissipated as heat. (Lower) The physical mechanism of the quenching: the excited Chl transfers energy to a carotenoid S1 state, which dissipates it by decaying to the ground state on a timescale of a few picoseconds.

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