[Alternative transcripts from POLRMT responsible for synthesis of nuclear RNA polymerase IV].

Analysis of ESTs corresponding to human for mitochondrial RNA polymerase suggested the existence of an alternative transcript. This mRNA, which contained 225 bp of the intron 1 proximal region, was detected in HeLa cells by reverse transcription–polymerase chain reaction. Alternative transcripts containing intron 1 were also found in mouse and rat cells. Moreover, a third transcript with an additional exon (142 bp), which originates from intron 2, was observed in mouse cells. In contrast to the major mRNA coding for the full-size RNA polymerase, the alternative transcripts code for a protein truncated at the N end and devoid of the mitochondrial targeting signal. The protein occurs exclusively in the nucleus and corresponds to new nuclear RNA polymerase IV, which was recently identified.