Plant organellar DNA primase-helicase synthesizes RNA primers for organellar DNA polymerases using a unique recognition sequence

DNA primases recognize single-stranded DNA (ssDNA) sequences to synthesize RNA primers during lagging-strand replication. Arabidopsis thaliana encodes an ortholog of the DNA primase-helicase from bacteriophage T7, dubbed AtTwinkle, that localizes in chloroplasts and mitochondria. Herein, we report that AtTwinkle synthesizes RNA primers from a 5'-(G/C) GGA-3' template sequence. Within this sequence, the underlined nucleotides are cryptic, meaning that they are essential for template recognition but are not instructional during RNA synthesis. Thus, in contrast to all primases characterized to date, the sequence recognized by AtTwinkle requires two nucleotides (5'-GA-3') as a cryptic element. The divergent zinc finger binding domain (ZBD) of the primase module of AtTwinkle may be responsible for template sequence recognition. During oligoribonucleotide synthesis, AtTwinkle shows a strong preference for rCTP as its initial ribonucleotide and a moderate preference for rGMP or rCMP incorporation during elongation. RNA products synthetized by AtTwinkle are efficiently used as primers for plant organellar DNA polymerases. In sum, our data strongly suggest that AtTwinkle primes organellar DNA polymerases during lagging strand synthesis in plant mitochondria and chloroplast following a primase-mediated mechanism. This mechanism contrasts to laggingstrand DNA replication in metazoan mitochondria, in which transcripts synthesized by mitochondrial RNA polymerase prime mitochondrial DNA polymerase gamma.