Nature of steady-state and newly synthesized mitochondrial messenger ribonucleic acids in mouse liver and Ehrlich ascites tumor cells

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Abstract
The steady-state mitochondrial mRNA in Ehrlich ascites tumor cells and mouse liver were identified by the Northern blot analysis using nick-translated mt[mitochondrial]DNA and 32P-labeled c[complementary]DNA probes. The steady-state mRNA species were compared with the poly(A)-containing RNA synthesized in vitro in isolated mitoplasts. The isolated mitoplast system can efficiently transcribe almost all of the poly(A)-containing RNA detected in the steady-state RNA population. The mode of transcription and maturation of mitochondrial mRNA in different mouse tissues are identical. The results of Northern blot analyses suggest that there may be at least 2 different modes of mRNA maturation depending upon if the reading frames are interrupted by tRNA cistrons or not. mRNA for reading frames with adjacent tRNA cistrons downstream appear to be processed from very short-lived precursors. mRNA coded by adjacently located reading frames with no interrupting tRNA genes such as URF3-cyt ox III and URF5-cyt b are processed from relatively long-lived precursors. The in vitro pulse-labeling studies also show that almost all of the poly(A)-containing mRNA are transcribed at nearly identical rates, suggesting that the major regulation of mt gene expression may occur at the level of translation or mRNA decay. The present experiments have also identified a 1.85-kb [kilobase] poly(A)-containing RNA as the putative URF5 mRNA.