Species specificity in protein synthesis

The phenomenon of species specificity in poly U-directed cell-free protein synthesis (whereby combinations of ribosomes of one organism with supernatant enzymes from a different one are sometimes inactive) was studied in the case of the amino acid-incorporating systems prepared from Escherichia coli, Bacillus subtilis, Saccharomyces cerevisiae, castor bean seedlings and rat liver. These organisms could be classified into at least two groups, one including B. subtilis and E. coli, and the other S. cerevisiae, castor bean seedlings and rat liver. It appears that within each group, the combination of ribosomes and supernatant enzymes extracted from different organisms gave active synthesis, whereas the permutations of ribosomes and enzymes prepared from organisms belonging to the two different groups gave very little if any synthesis. The mechanism of incompatibility was analyzed in detail in the E. coli-castor bean seedlings combination. It was found that the inability to synthesize polypeptides was not due to any of the following factors: the synthesis of phenylalanyl-sRNA; the source of the phenylalanyl-sRNA used; the selective degradation of heterologous phenylalanyl-sRNA; the binding of phenylalanyl-sRNA to ribosomes; the presence of specific inhibitory activities. The tentative conclusion is drawn that failure to observe protein synthesis in mixed systems is due to the lack of interaction between ribosomes of a given size (70 or 80 s) and polymerizing enzymes extracted from organisms containing ribosomes of different size.