Analysis of developing maize plastids reveals two mRNA stability classes correlating with RNA polymerase type.

The plastid genome is transcribed by two distinct RNA polymerases, the PEP encoded by the plastid genome and the NEP encoded in the nucleus. Initial models of plastid transcription held that the NEP is responsible for the transcription of housekeeping genes needed early in development, and that the PEP transcribes genes required for photosynthesis. Recently, this model was challenged by the discovery that all plastid genes are transcribed by NEP in PEP-deficient tobacco plastids, suggesting that mRNA turnover may have a strong role in previously observed transcription patterns. In this study, we provide evidence that the NEP enzyme level decreases as plastids mature. In contrast, production of mRNAs by NEP increases as plastids mature, yet their accumulations remain constant. These results suggest that as plastids mature NEP may become more active, and that mRNA turnover varies between transcripts synthesized by NEP and PEP.