Roles of chloroplast RNA polymerase sigma factors in chloroplast development and stress response in higher plants

Chloroplast transcription in higher plants is performed by two types of RNA polymerases, plastid-encoded RNA polymerase (PEP) and nuclear-encoded RNA polymerase (NEP). PEP is a eubacteria-type multisubunit enzyme whose catalytic core subunits are encoded by the chloroplast genome, whereas NEP is the nuclear encoded T7 phage-type single subunit enzyme. PEP is critical for the biogenesis and maintenance of chloroplasts, and is finely tuned by the nuclear encoded sigma subunits. Of the six Arabidopsis sigma subunits, SIG2 is involved in the transcription of several chloroplast tRNA genes, including trnE encoding tRNA-Glu. SIG2 possibly couples translation and pigment synthesis in chloroplasts. On the other hand, SIG5 is induced by various stresses and contributes to repair of damaged photosystem II (PSII) through transcription of the psbD and psbC genes. Thus target genes and the physiological role of each sigma subunit are becoming clearer.