1. Decapped tobacco mosaic virus (TMV) RNA and rabbit globin mRNA were prepared 
by enzymic treatment of RNAs with nucleotide pyrophosphatase purified from 
potato. The extent of removal of 5'-terminal 7-methylguanosine 5'-monophosphate 
(m7GMP) from TMV RNA was at least 97% as estimated by labeling of the 5' termini 
in vitro with S-adenosyl[methyl-3H]methionine catalysed by vaccinia virus 
methyltransferases. 2. The effect of enzymic decapping was compared with the 
effect of cap analogs on mRNAs translation in a nuclease-treated rabbit 
reticulocyte lysate and in a wheat germ extract. When translation was studied at 
low K+ concentration, little or no dependence on 5'-terminal 7-methylguanosine 
was found with either cell-free system. The importance of the 5'-terminal cap 
for the efficient translation of TMV RNA and globin mRNA increased as the 
concentration of K+ in a protein-synthesis system was raised. In a reticulocyte 
lysate analogs and enzymic decapping had a similar effect on translation. In a 
wheat germ extract, mRNA decapping resulted in a more pronounced decrease of 
mRNA activity, presumably due to the increased susceptibility of decapped mRNAs 
to the nucleases present in this protein synthesis system. 3. The requirement 
for a 5'-terminal cap was similar for the synthesis of 130,000-Mr and 165,000-Mr 
polypeptides coded by TMV RNA. This indicates that both proteins may be 
initiated at the common site close to the 5' terminus.