4.5 Article

Evidence that chlorophyll f functions solely as an antenna pigment in far-red-light photosystem I from Fischerella thermalis PCC 7521

Journal

BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS
Volume 1861, Issue 5-6, Pages -

Publisher

ELSEVIER
DOI: 10.1016/j.bbabio.2020.148184

Keywords

Electron transfer; Photosystem I; Chlorophyll f; Femtosecond absorption spectroscopy; Far-red light photoacclimation (FaRLiP); Photosynthesis; Cyanobacteria

Funding

  1. Russian Science Foundation (RSF)
  2. US National Science Foundation (NSF)
  3. NSF [MCB-1613022]
  4. [RSF 19-14-00366]
  5. [RSF 17-14-01323]
  6. Russian Science Foundation [19-14-00366] Funding Source: Russian Science Foundation

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The Photosystem I (PSI) reaction center in cyanobacteria is comprised of similar to 96 chlorophyll (Chl) molecules, including six specialized Chl molecules denoted Chl1A/Chl1B (P-700), Chl2A/Chl2B, and Chl3A/Chl3B that are arranged in two branches and function in primary charge separation. It has recently been proposed that PSI from Chroococcidiopsis thermalis (Nurnberg et al. (2018) Science 360, 1210-1213) and Fischerella thermalis PCC 7521 (Hastings et al. (2019) Biochim. Biophys. Acta 1860, 452-460) contain Chl f in the positions Chl2A/Chl2B. We tested this proposal by exciting RCs from white-light grown (WL-PSI) and far-red light grown (FRL-PSI) F. thermalis PCC 7521 with femtosecond pulses and analyzing the optical dynamics. If Chl f were in the position Chl2A/Chl2B in FRL-PSI, excitation at 740 nm should have produced the charge-separated state P700(+)A(0)(-) followed by electron transfer to A(1) with a tau of <= 25 ps. Instead, it takes similar to 230 ps for the charge-separated state to develop because the excitation migrates uphill from Chl f in the antenna to the trapping center. Further, we observe a strong electrochromic shift at 685 nm in the final P700+A1(-) spectrum that can only be explained if Chl a is in the positions Chl2A/Chl2B. Similar arguments rule out the presence of Chl f in the positions Chl3A/Chl3B; hence, Chl f is likely to function solely as an antenna pigment in FRL-PSI. We additionally report the presence of an excitonically coupled homo- or heterodimer of Chl f absorbing around 790 nm that is kinetically independent of the Chl f population that absorbs around 740 nm.

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