Post-transcriptional addition of polyadenylic acid to mitochondrial RNA by a cordycepin-insensitive process

Abstract

An RNA fraction in HeLa cell mitochondria has previously been shown to contain a small polyadenylic acid sequence (56 nucleotides) and to behave like messenger RNA. This fraction can be resolved into approximately eight distinct species which may be the messages for the mitochondrial-specified proteins. The poly(A) sequence (4 S by electrophoresis) attached to mitochondrial-specific RNA is shown to be added post-transcriptionally. The kinetics of poly(A) addition in the presence of actinomycin indicate that the half-life of this process is approximately one hour. The enzymatic process that adds poly(A) to mitochondrial RNA appears to be distinct from the nuclear enzyme with similar function. Cordycepin, in contrast to its effect in the nucleus, is found to inhibit the transcription of mitochondrial DNA, but to have no direct effect on the synthesis and addition of mitochondrial poly(A). Little or no completely “free” poly(A) is apparent in these experiments, suggesting that the polymerization of the adenine residues takes place on existing RNA. In the presence of ethidium, poly(A) addition becomes aberrant and most of the newly synthesized poly(A) is attached to very small pieces of RNA.