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Primary transcripts in higher plant cell organelles undergo a series of essential RNA processing steps, including 5′- and 3′-terminal maturation, intron splicing and RNA editing. Most of the protein factors involved in these RNA processing mechanisms are still unknown. Also, little is known about the environmental and developmental factors regulating RNA processing events in plastids. Here we have tested the influence of the environmental factor temperature on RNA processing in the ndhB mRNA from tobacco plastids. We find that, whereas temperature increase to 37 °C fully inhibits RNA editing at only one out of nine sites in the tobacco ndhB transcript, further temperature increase to 42 °C selectively blocks editing at two additional sites. As these temperature effects are strictly site-specific, our findings provide evidence for the existence of site-specific editing factors which exhibit differential …