Expression of the mitochondrial genome in HeLa cells : I. Properties of the discrete RNA components from the mitochondrial fraction

The 16 s, 12 s and 4 s RNA components from the mitochondrial fraction of HeLa cells have been analyzed as to their sedimentation and electrophoretic properties, kinetics of labeling, metabolic stability, response to inhibitors of RNA synthesis, nucleotide composition and methylation level. After denaturation by heat-formaldehyde treatment, the 16 s and 12 s species sediment in sucrose gradients in the presence of formaldehyde as homogeneous components running behind 18 s RNA. Relative to the 28 s and 18 s RNA markers, the 16 s and 12 s RNA components behave in polyacrylamide gel electrophoresis as expected, in the absence of conformational influences, for a species slightly larger than 18 s RNA (i.e. with a molecular weight about 0.7 × 10⁶) and respectively, for a species with a molecular weight of about 0.4 × 10⁶. The 16 s and 12 s RNA correspond to the “21 s” and “12 s” electrophoretic components previously described in HeLa cells. However, in contrast to what has been reported for the latter components, the 16 s and 12 s RNA have been found to be methylated; furthermore, these species appear to have a considerably longer half-life than previously surmised on the basis of their behaviour in the presence of ethidium bromide and evidence is presented suggesting that they are synthesised in equimolar amounts.