The three major structural proteins of vesicular stomatitis virus, M (mol. wt. 29000), N (mol. wt. 50000) and G (mol. wt. 69000) are synthesized at non-equimolar rates. The possibility of translational control of VSV protein synthesis by examining the size distributions of polysomes involved in the synthesis of these three proteins has been tested. The strategy was to label the nascent peptide chains with radioactive amino acids and to analyse separately membrane-bound polysomes, involved predominantly in synthesis of the G protein, and soluble polysomes, involved predominantly in the synthesis of the M and N proteins. Analysis by sucrose gradient centrifugation indicated that the G protein was synthesized on membrane-bound polysomes having a mean size of 8 ribosomes per polysome. The M and N proteins were synthesized on soluble polysomes having a mean size of 5 ribosomes per polysome. Both membrane-bound and soluble polysomes had a maximal size of approximately 15 ribosomes per poly-some. The distribution of M and N nascent chains within the soluble polysome gradient was determined by tryptic peptide analysis. The M nascent chains were found to be associated predominantly with the smaller polysomes. Thus, the data indicate that the average size of the polysomes involved in the synthesis of the M, N and G proteins reflects the mol. wt. of these proteins. This suggests that the relative rates of synthesis of the M, N and G proteins are not the outcome of controls at the translational level.


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