Sea urchin mtDBP is a two-faced transcription termination factor with a biased polarity depending on the RNA polymerase

The sea urchin mitochondrial displacement (D)-loop binding protein mtDBP ha s been previously identified and cloned. The polypeptide (348 amino acids) displays a significant homology with the human mitochondrial transcription termination factor mTERF. This similarity, and the observation that the 3' ends of mitochondrial RNAs coded by opposite strands mapped in corresponden ce of mtDBP-binding sites, suggested that mtDBP could function as transcrip tion termination factor in sea urchin mitochondria. To investigate such a r ole we tested the capability of mtDBP bound to its target sequence in the m ain non-coding region to affect RNA elongation by mitochondrial and bacteri ophage T3 and T7 RNA polymerases. We show that mtDBP was able to terminate transcription bidirectionally when initiated by human mitochondrial RNA pol ymerase but only unidirectionally when initiated by T3 or T7 RNA polymerase s. Time-course experiments indicated that mtDBP promotes true transcription termination rather than transcription pausing. These results indicate that mtDBP is able to function as a bipolar transcription termination factor in sea urchin mitochondria. The functional significance of such an activity c ould be linked to the previously proposed dual role of the protein in modul ating mitochondrial DNA transcription and replication.