Messenger RNA in animal cells.

It is difficult to believe that only 2 years ago, the very existence of messenger RNAs in animal cells was doubtful, and no definitive example of an isolated and purified mRNA could be given. Since that time, the mRNAs for globin (from rabbit, man, mouse, duck, and guinea pig), lens crystallin (chick and cow), ovalbumin, myosin, and silk fibroin have been isolated and their identities confirmed, and histone, protamine, and tubulin mRNAs have been putatively identified. It can be stated with a degree of certainty that if a laboratory has access to a tissue making large amounts of a single protein, and an assay for that protein which is both unique and sensitive, it should be possible to isolate and characterize the mRNA for that protein. Messenger RNA was first identified in bacterial systems by its rapid labelling with radioisotopes, a rapid turnover relative to ribosomal RNA and 'DNA-like' base composition (Gros et al, 1961; Jacob and Monod, 1961). Although similar criteria have been applied to mRNA in some mammalian tissues, such as liver, in general mRNA in a differentiating cell need not differ in labelling or base composition from other RNA species. Therefore attempts were made to obtain stimulation of the synthesis of a specific protein in a mixed cell-free system. It is essential that the cell-free protein synthesis system used should not normally be capable of synthesizing the protein, for there are many factors which can stimulate endogenous protein activity apart from mRNA. An unambiguous assay system for the protein is also necessary, to distinguish it from endogenous protein synthesis characteristic of the cell-free system.