Why a chloroplast needs its own genome tethered to the thylakoid membrane -- Co-location for Redox Regulation

Abstract

A chloroplast is a subcellular organelle of photosynthesis in plant and algal cells. A chloroplast genome encodes proteins of the photosynthetic electron transport chain and ribosomal proteins required to express them. Chloroplast-encoded photosynthetic proteins are mostly intrinsic to the chloroplast thylakoid membrane where they drive vectorial electron and proton transport. There they function in close contact with proteins whose precursors are encoded in the cell nucleus for cytosolic synthesis, subsequent processing, and import into the chloroplast. The protein complexes of photosynthetic electron transport thus contain subunits with one of two quite different sites of synthesis. If most chloroplast proteins result from expression of nuclear genes then why not all? What selective pressure accounts for the persistence of the chloroplast genome? One proposal is that photosynthetic electron transport itself governs expression of genes for its own components: co-location of chloroplast genes with their gene products allows redox regulation of gene expression, thereby resulting in self-adjustment of protein stoichiometry in response to environmental change. This hypothesis posits Co-Location for Redox Regulation, termed CoRR, as the primary reason for the retention of genomes in both photosynthetic chloroplasts and respiring mitochondria. I propose that redox regulation affects all stages of chloroplast gene expression and that this integrated control is mediated by a chloroplast mesosome or nucleoid - a structure that tethers chloroplast DNA to the thylakoid.