Evidence for non-uniform translation of individual polypeptides in rat liver mitochondria

[35S]Methionine labelled mitochondrial translation products were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) in the presence of 8 M urea. Following fluorography and scanning, the synthesis time of major polypeptides was determined. It was found that: (1) from 9 to 16 min are required to synthesize various complete polypeptides whose size differs by a factor of more than 5. (2) The specific radioactivity of complete polypeptides with electrophoretic mobilities corresponding to 6 kDa, 16 kDa and 31.5 kDa remains constant during the first 10 min of labelling and attains its final value only after a certain lag-period. (3) In the electrophoretic pattern at least two bands (6.3 kDa and 7.2 kDa) among other bands are nascent, and they accumulate in amounts comparable with that of the complete ones. These data are in agreement with the model of non-uniform elongation rate with a sharp delay (pause) during translation of internal mRNA regions.