A Window into the Early Steps in Plastid Evolution

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

INTELLECTUAL MERIT The proposed work addresses the question of the way in which the photosynthetic organelle, the chloroplast, evolved. The organism used in these studies is the thecate protist Paulinella chromatophora, which contains what visually resembles a cyanobacterium within its cytoplasm, although this internal "cyanobacterium" is really an organelle called a chromatophore. Even though the chromatophore does not look like a chloroplast, it performs many of the same functions as chloroplasts, capturing sunlight to drive photosynthesis, which provides the protist host cell with energy and carbon backbones for growth. The chromatophore contains its own genome (DNA), like a chloroplast, but the size of this genome is 5-10 times more than what is found in most chloroplasts, but much less than what is found in cyanobacteria. These findings suggest that the chromatophore is an organelle derived from an endosymbiotic cyanobacterium that is in the process of evolving into a chloroplast, and as such it can reveal important information about mechanisms critical for the early events in chloroplast evolution. We recently discovered that a number of proteins important for the function of the chromatophore are synthesized outside of the chromatophore and in the cytoplasm of the host protist cell, and must be transported into the organelle where they function (e.g. PsaE, a protein that functions in photosynthesis). The major specific thrust of our studies is to identify those proteins and to develop an understanding of how they move from the cytoplasm of the host protist into the nascent chloroplast, where they assemble into large functional complexes. BROADER IMPACTS In a broad sense, this work is critical to our understanding of both the processes by which cellular organelles evolved and the ways in which two organisms can come together and integrate their functions into a highly efficient single organism. The result of this work and the concepts that grow out of those results will have broad intellectual impact in that they will: (1) shed light on early events associated with the transport of proteins into developing organelles; (2) suggest which proteins are most suited (based on their biophysical properties) to enter developing chloroplasts through systems that already exist in the host organism; (3) allow for the identification of proteins that may require more extensive tailoring before they can assume the right conformation to enter organelles (they may have to enter the chloroplast through a more specialized system that develops over evolutionary time); (4) provide a broader understanding of the development of regulatory events that have integrated the synthesis of proteins in the cytoplasm of the cell with those synthesized in the chloroplast; (5) suggest new ways in which proteins can be engineered to get them into specific subcellular compartments, which may be important for aspects of synthetic biology and the tailoring cells to make specific products such as biofuels; (6) train graduate students to perform experiments that test evolutionary concepts using molecular and biochemical tools and techniques; (7) be incorporated into an undergraduate course that is being taught at Stanford University that focuses on the functionality, evolution and regulation of photosynthetic processes ("From Photosynthesis to Biofuels").