CHARACTERISTICS AND COMPOSITION OF RNA CODING UNITS

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Abstract
The activity of polyuridylic acid in directing polyphenylalanine synthesis was found to vary with the molecular weight of polyuridylic acid; longer polynucleotide chains were more active than shorter ones. Evidence was presented which suggested that one molecule of polyuridylic acid may direct the synthesis of a number of molecules of polyphenylalanine. Randomly mixed polynucleotides as well as "homopolynucleotides" were used to direct cell-free amino acid incorporation. Using this technique the nucleotide compositions of RNA coding units corresponding to 15 amino acids were determined. Characteristics of the code such as degeneracy, the existence of nonsense words, the minimum coding ratio, and the universality of the code were investigated. Two coding units corresponding to leucine were found; thus, part of the code was shown to be degenerate. Certain nucleotide sequences in a polynucleotide did not code for any amino acid: therefore, the presence of nonsense coding units was suggested. The minimum number of nucleotides per coding units appeared to be three. Comparison between the composition of RNA coding units in E. coli and amino acid replacement data in tabacco mosaic virus suggested that at least part of the code may be universal.