The influence of viral coding sequences on the efficiency of internal initiation of translation of cardiovirus RNAs.

Since internal initiation of translation of cardiovirus RNAs requires the approximately 450-nt segment of the viral genome immediately upstream of the authentic initiation codon for viral polyprotein synthesis, the question arises as to whether the immediately adjacent sequences, the start of the polyprotein coding region, also influence the efficiency of internal initiation. Therefore, a variety of constructs derived from encephalomyocarditis virus and Theiler's murine encephalomyelitis virus retaining various lengths of viral coding sequence were translated in rabbit reticulocyte lysates. Efficient internal initiation showed no requirement specifically for viral coding sequences, but on the other hand certain motifs, notably G-rich sequences, located immediately downstream of the initiation codon were highly inhibitory. These results suggest a possible explanation for the evolution of cardiovirus polyproteins lacking N-terminal myristylation signals and are also pertinent to the design of constructs in which the cardiovirus internal ribosome entry signal is used to drive the expression of a reporter cistron.