The EMS77 Nuclear Gene Encodes PTAC7, An Component Of The Transcriptionally Active Plastid Chromosome, Which Is Essential For Chloroplast Biogenesis

Chloroplasts are specialized organelles found in plant cells and green algae that conduct photosynthesis. Besides photosynthesis, chloroplasts are also involved with other various biological events:including starch, fatty acid, amino acids, pigment, nucleic acids and protein synthesis in cells and so on.Chloroplasts are genetic semiautonomous organelles and have their own genome. In Arabidopsis, chloroplasts contain 100 genes,which only encode about 80 proteins(Martin andHerrmann 1998). Importantly, chloroplast-localized protein assemblies contain not only plastid-encoded subunits, but also nuclear-encoded proteins(McFadden 1999). The vast majority of the greater than 2,000 proteins that have functions in the chloroplast are encoded by nuclear genes, translated in the cytosol, and subsequently imported into the chloroplast (Abdallah et al., 2000).Therefore, coordinated expression of these genes in chloroplast required to organize between plastids and the nucleus. Consistent with the need for intercompartmental communication, expression of plastid-encoded genes is regulated by a battery of nuclear-encoded proteins.The expression of plastid-encoded gene is carried out at least by two RNA polymerases of different origin (Hu and Bogorad, 1990; Igloiand Ko" ssel,1992; Lerbs-Mache,1993; Pfannschmidt and Link,1994; Liere and Maliga,1999; Shiina et al.,2005). One is the plastid-encoded polymerase (PEP), which is a multi-subunit enzyme complex and has been retained from the ancestral endosymbiont. The other is nuclear-encoded polymerase (NEP), phage-type, made of a single subunit. Although genes transcribed by both NEP and PEP polymerases exist, in general, a sequential action occurs:genes with NEP promoters are transcribed early in chloroplast development and are involved in housekeeping functions, primarily constituting the plastid genetic machinery, including the subunits of PEP. Thereafter PEP is involved in the expression of photosynthesis-related genes (Lopez-Juez et al.,2006; Hajdukiewicz et al.,1997).Since PEP accounts for the transcription of photosynthetic genes and the correct expression of photosynthetic genes is tightly connection with chloroplast development, it is essential to maintain the PEP activity for chloroplast development, and any factors which affect the activity, may block chloroplast development.Although the core subunits of PEP complex are plastid-encoded, the transcription activity of PEP need the participation of nuclear-encoded factors. Several reports indicated that subunits of the PEP core are present in two plastid protein preparations, the transcriptionally active chromosome (TAC) and the soluble RNA polymerase (sRNAP) (Igloi and Kossel,1992; Krause et al.,2000; Pfalz et al.,2006). Many efforts have been made to purify and identify these proteins from unicellular algae (Hallick et al.,1976; Rushlow and Hallick,1982; Narita et al.,1985) and higher plants (Briat and Mache,1980; Reiss and Link,1985; Krupinska and Falk,1994; Pfannschmidt et al., 2000; Suzuki et al.,2004) since 1970's. Recently, Pfalz and his colleage (2006) purified TACs from Arabidopsis and mustard(Sinapis alba), and identified 18 new components, called plastid transcriptionally active chromosome proteins (pTACs).Here, we characterized a gene EMS77, which encodes a compontent of the transcriptionally active plastid chromosome complex, pTAC7. The two null mutants of the pTAC7 gene displayed albino phenotype and the chloroplast development was arrested at early stage. The expression pattern of plastid genes in the null mutants resembles those of theΔrpo mutants, demonstrating that the pTAC7 regulated plastid gene expression as a functional compontent of the TACs, and is essential for chloroplast development at early stage.