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Our previous data suggested that translation in an EMC virus RNA-programmed cell-free system from Krebs-2 cells is initiated predominantly at a single site and that the earliest amino acid sequences synthesized correspond to non-structural ‘leader’ polypeptides p14 and p12 [(1982) FEBS Lett. 141, 153–156]. Here, polypeptides p14 and p12 were labelled in vitro by tritiated amino acids, isolated and subjected to automated Edman degradation. Both polypeptides (after the loss of the N-terminal methionine) were shown to contain alanine in position 1 and glutamic acid in positions 5 and 7. These and other data demonstrate that p14 and p12 share a common N-terminal sequence. This sequence coincides precisely with the N-terminus of EMC virus polyprotein sequence deduced from the primary structure of the viral genome [(1984) Nucleic Acids Res., in press]. Thus, the single initiation site operating in our translation system corresponds to the start of the polyprotein molecule.