IF3 - interference factors : protein factors in Escherichia coli controlling initiation of mRNA translation

Abstract

Summary Initiation factor IF3 activity can be separated into subfractions differing by their relative activity for various mRNA templates. In most cases, it can be shown that the template specificity of these subfractions results from the presence of other protein factors, having no IF3 activity by themselves, but which interfere with IF3 activity. Purified IF3 is active for both MS2 and T4 mRNA, but on MS2 RNA it stimulates selectively initiation of the coat protein cistron. Addition of interference factor i to this standard IF3, blocks initiation at the MS2 coat protein cistron, but stimulates it at the MS2 synhetase cistron. Moreover, this protein of MW 74,0000, becomes after RNA phage infection a subunit of the RNA replicase. Factor i stimulates also T7 mRNA translation. On T4 mRNA, it stimulates the translation of certain cistrons, while inhibiting that of others. In addition to factor i, other interference factors have been detected. Factor J, for example, inhibits MS2 coat and synthetase cistrons ; it inhibits T7 mRNA translation and produces a higher inhibition of early T4 mRNA than late T4 mRNA. Another interference factor inhibits late T4 mRNA more than MS2 RNA. In combination with IF3, these proteins give a series of initiation factors with very different template specificities. They bind to ribosomes and IF3 and confer to the ribosome initiation complex a selective affinity for certain initiation signals. After T4 infection, the changes in ribosome template specificity may result from a change either in IF3 or in an interference factor. Immunological and biochemical data indicate that IF3 is unchanged. However interference factor i is chemically modified after T4 infection. Infection of E. coli F+ strains by phage T7 results in a rapid loss of IF3 from the cell, which may be responsible for the absence of late protein synthesis and for the abortive T7 infection in these strains. This does not happen in E. coli F− strains. In stationary phase E. coli, there is a partial loss of IF3 : this decreases the ratio of IF3 to interference factor i and alters the relative translation of T4 and MS2 RNA. From these examples, it is concluded that IF3-interference factors participate in the control of mRNA translation.