The reduced plastid-encoded polymerase-dependent plastid gene expression leads to the delayed greening of the Arabidopsis fln2 mutant.

In Arabidopsis leaf coloration mutants, the delayed greening phenomenon is common. Nonetheless, the mechanism remains largely elusive. Here, a delayed greening mutant fln2-4 of FLN2 (Fructokinase-Like Protein2) was studied. FLN2 is one component of Transcriptionally Active Chromosome (TAC) complex which is thought to contain the complete plastid-encoded polymerase (PEP). fln2-4 displayed albino phenotype on medium without sucrose. The PEP-dependent plastid gene expression and chloroplast development were inhibited in fln2-4. Besides interacting with thioredoxin z (TRX z), we identified that FLN2 interacted with another two members of TAC complex in yeast including its homologous protein FLN1 (Fructokinase-Like Protein1) and pTAC5. This indicates that FLN2 functions in regulation of PEP activity associated with these TAC components. fln2-4 exhibited delayed greening on sucrose-containing medium. Comparison of the PEP-dependent gene expression among two complete albino mutants (trx z and ptac14), two yellow mutants (ecb2-2 and ys1) and the fln2-4 showed that fln2-4 remains partial PEP activity. FLN2 and FLN1 are the target proteins of TRX z involved in affecting the PEP activity. Together with the data that FLN1 could interact with itself in yeast, FLN1 may form a homodimer to replace FLN1-FLN2 as the TRX z target in redox pathway for maintaining partial PEP activity in fln2-4. We proposed the partial PEP activity in the fln2 mutant allowed plastids to develop into fully functional chloroplasts when exogenous sucrose was supplied, and finally the mutants exhibited green phenotype.

TAC7, an essential component of the plastid transcriptionally active chromosome complex, interacts with FLN1, TAC10, TAC12 and TAC14 to regulate chloroplast gene expression in Arabidopsis thaliana.

Transcriptionally active chromosome (TAC) is a fraction of protein/DNA complexes with RNA polymerase activity in the plastid. However, the function of most TAC proteins remains unknown. Here, we isolated two allelic mutants of the gene for a TAC component, TAC7, and performed functional analysis in plastid gene expression and chloroplast development in Arabidopsis. tac7-1 is a mutant with a premature translation termination isolated from a population treated with ethyl methane sulfonate, and tac7-2 is a transfer-DNA tagging mutant. Both of them showed an albino phenotype when grown under normal light conditions, and a few appressed membranes were observed inside the defective chloroplasts. These data indicate that TAC7 is important for thylakoid biogenesis. The TAC7 gene encodes an uncharacterized 161 amino acids polypeptide localized in chloroplast. The transcriptional levels of plastid-encoded polymerase (PEP)-dependent genes were downregulated in tac7-2, suggesting that PEP activity was decreased in the mutant. Yeast two-hybrid assay shows that TAC7 can interact with the four TAC components including FLN1, TAC10, TAC12 and TAC14 which are involved in redox state changes, phosphorylation processes and phytochrome-dependent light signaling, respectively, These data indicate that TAC7 plays an important role for TAC to regulate PEP-dependent chloroplast gene expression and chloroplast development.