A eubacterial-type RNA polymerase (PEP) in higher plants plays crucial roles in chloroplast development. Six regulatory sigma factor genes (SIG1 to SIG6) encoded in the nuclear DNA are found in Arabidpopsis, however, the definite gene specificity of each sigma factor is still unknown. We recently identified an Arabidopsis recessive pale-green mutant sig2-1 (formerly named abc1) in which T-DNA is inserted in SIG2 (formerly named sigB; At1g08540). This mutant could develop almost normal etioplasts under dark conditions but barely develop the small chloroplasts with poor thylakoid membranes and stacked lamellar under light conditions. In the sig2-1 mutant, both chlorophyll and photosynthetic and photosynthesis-related proteins were remarkably reduced. However, amounts of mRNAs for these plastid-encoded proteins were not altered. Further analyses revealed that several plastid-encoded tRNAs including trnE-UUC were drastically reduced in the mutant. The plastid-encoded tRNA-Glu is indispensable for not only protein synthesis (translation) but also tetrapyrrole (chlorophyll and heme) biogenesis. We could assign -35 like and -10 like sequences to upstream of these SIG2-dependent tRNA genes, which are conserved in Arabidopsis, tobacco, spinach, rice and maize. On the other hand, transcripts of nucleus-encoded T7 phage-type RNA polymerase (NEP)-dependent genes were steadily accumulated in this mutant. These results indicate that SIG2 plays crucial roles in chloroplast development mediated by expression of some plastid-encoded tRNAs and negative regulation of NEP.
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