Regulatory and Functional Characterization of Modular Photoprotective Proteins in the Context of Cyanobacterial Ecology and Evolution

Michigan State University, East Lansing MI

Investigators

Abstract

Photosynthetic organisms convert light energy to chemical energy in the process of photosynthesis. However, absorption of light in excess of that which can be used for photosynthesis can lead to light-induced damage. Thus, photoprotective mechanisms have evolved in photosynthetic organisms to deal with excess light absorption or stressful conditions, including fluctuating light. Photoprotective mechanisms used by cyanobacteria have only recently begun to be characterized. A key factor involved in cyanobacterial photoprotection is the Orange Carotenoid Protein (OCP) that works together with the Fluorescence Recovery Protein (FRP) to convert excess light energy to heat to protect the photosynthetic reaction centers from photodamage, thereby maintaining photosynthetic efficiency and productivity. The proposed work will contribute to a greater understanding of the range of variants of these proteins that are present in ecologically distinct cyanobacteria. In addition to training students and postdoctoral scientists in diverse methods, broader impacts of the research include understanding the mechanisms used by cyanobacteria to protect themselves from excess light absorption and associated light-induced damage. Such knowledge will enhance efforts to increase the efficiency of light capture and reduce photodamage in bioenergy or production strains of cyanobacteria. The project also includes structured mentoring in hands-on research for undergraduates from groups underrepresented in the sciences and participation in an annual Microbiology Day at a local science museum targeting primary and middle school students. In contrast to those of plants, the photoprotective mechanisms of cyanobacteria have only recently begun to be characterized. One of the most prevalent, the OCP/FRP (Orange Carotenoid Protein/Fluorescence Recovery Protein) system responds to high light by dissipating excess energy captured by the light harvesting antenna (phycobilisome or PBS). The OCP is a soluble, 35 kDa protein that binds a single carotenoid molecule. It is the only known photoactive protein that uses a carotenoid as its sole chromophore. The absorption of blue-green light causes the OCP to convert from a dark stable orange form, OCPO, to a light-activated red form, OCPR. OCPR directly participates in photoprotection by binding to the PBS. Recovery from the energy-dissipative state is catalyzed by the FRP. Structurally, the OCP consists of two domains, one serving as a sensor/regulatory domain whereas the other functions directly in energy dissipation. We have identified a family of paralogous genes encoding the separate domains in Fremyella diplosiphon and hypothesize that they reflect the evolution of the OCP and that they provide flexibility in tuning photoprotection in Fremyella under dynamic environmental conditions. A combination of in vivo approaches in Fremyella and in vitro analysis of the isolated proteins will be used in conjunction with bioinformatics to test these hypotheses. In addition to training students and postdoctoral scientists in diverse methods, broader impacts of the research include an in-depth understanding of the flexibility of photoprotective mechanisms that may be useful in bioengineering of cyanobacteria.