The Evolutionary Origin Of Translocons On The Inner Envelope Of Chloroplasts & The Cloning Of A Nuclear Encoded Chloroplast Gene DPC1 And Its Functional Analysis In Oxidative Stress Tolerance

The process of importing nuclear encoded proteins into chloroplasts is mediated by the TOC and TIC complex(Translocons on the Outer/Inner Envelope of Chloroplasts).The ancestor of the TOC complex was formed by pre-existing proteins from the cyanobacterial ancestor;other proteins recruited from the host cell or cyanobacterial ancestor were subseqently integrated into the complex.However,little is known about the origin of the TIC complex.In this work,the origin of the TIC complex was investigated through one of its channel proteins,AtTic21.Phylogenetic analysis suggested that AtTic21 is conserved in photosynthetic organisms.AtTic21 showed 33%sequence identity to a Synechocystis protein SynTic21.The successful genetic complementation of an AtTic21 knockout mutant by SynTic21 plus the transit peptide coding sequence of AtTic21 suggested that SynTic21 is an ortholog of AtTic21.The sequence and functional conservation between SynTic21 and AtTic21 suggested that the TIC complex shares a similar evolutionary origin to the TOC complex.To gain insight into the process of chloroplast development and photosynthesis,we characterized the damaged photosynthesis capacity 1(dpc1) mutant that showed pale green phenotype from a population of activation-tagged Arabidopsis lines.The rosette leaves of dpc1,especially the juvenile,showed pale green phenotype as the result of the decreased levels of chlorophyll and a higher ratio of chlorophyll a versus chlorophyll b. Histochemical staining and hydrogen peroxide bleaching results indicated that there are accumulations of reactive oxygen species and lower photo-oxidative stress tolerance in leaves of dpc1.Moreover,the leaves of dpc1 accumulate more anthocyanins than that of wild type plants when floated on water under light.The values of P700 and Fv/Fm indicated that the photosynthetic capacities of PSâ… and PSâ…¡were damaged in dpc1. Microscopy results of dpc1 indicated that the ultrastructure,population and distribution were unaltered.The corresponding gene DPC1 was cloned and it encodes a protein that consists of ten pentratricopeptide repeat domains and one small mutS-related domain. Protein fusion experiment indicated that DPC1 localized into chloroplast.Expression pattern analyses indicated that DPC1 expressed mainly confined green tissues.We concluded that DPC1 is essential in chloroplast development,photosynthesis maintaining and photo-oxidative stress tolerance.