Primer RNA

Primer RNA is RNA that initiates DNA synthesis. Primers are required for DNA synthesis because no known DNA polymerase is able to initiate polynucleotide synthesis. Edited transcriptional RNA is used to initiate DNA synthesis in some phage and in metazoan mitochondria. Transfer RNAs are used by retroviral reverse transcriptases to initiate replication of retroviral genomes. Bacteria, archaea, and eukaryotes use an enzyme called primase to synthesize short-lived oligonucleotides used only during DNA replication. These primases synthesize a primer RNA once on each leading strand template to initiate DNA synthesis and repeatedly on the lagging strand template to initiate Okazaki fragment synthesis. Primase initiate synthesis from specific trinucleotides that differ according to the organism's phylogeny. Once the specific purine-rich diribonucleotide has been synthesized complementary to the 5' and central nucleotides of the initiation trinucleotide, the rest of the primer sequence is synthesized in a template-dependent manner. In bacteria, the process of removing the primers is not directly coupled to discontinuous synthesis and involves such enzymes as RNase H and the 5'-exonuclease of DNA polymerase I.