LAUSR

RNA polymerase transcriptional pausing represents a major checkpoint in transcription in bacteria and metazoans, but it is unknown whether this phenomenon occurs in plant organelles. Here, we report that transcriptional pausing occurs in chloroplasts. We found that mTERF5 specifically and positively regulates the transcription of chloroplast psbEFLJ (encoding four key subunits of photosystem II) in Arabidopsis thaliana. mTERF5 causes the plastid-encoded RNA polymerase (PEP) complex to pause at psbEFLJ by binding to the +30 to +51 position of double-stranded DNA. Moreover, mTERF5 interacts with pTAC6, an essential subunit of the PEP complex, although pTAC6 is not involved in the transcriptional pausing at psbEFLJ. mTERF5 recruits additional pTAC6 to the transcriptionally paused region of psbEFLJ and this recruited pTAC6 can be assembled into the PEP complex to regulate psbEFLJ transcription. Our findings shed light on the role of transcriptional pausing in chloroplast transcription in plants.