The RNA-binding site of bacteriophage Qbeta coat protein.

The coat proteins of the RNA bacteriophages Qbeta and MS2 are specific RNA binding proteins. Although they possess common tertiary structures, they bind different RNA stem loops and thus provide useful models of specific protein-RNA recognition. Although the RNA-binding site of MS2 coat protein has been extensively characterized previously, little is known about Qbeta. Here we describe the isolation of mutants that define the RNA-binding site of Qbeta coat protein, showing that, as with MS2, it resides on the surface of a large beta-sheet. Mutations are also described that convert Qbeta coat protein to the RNA binding specificity of MS2. The results of these and other studies indicate that, although they bind different RNAs, the binding sites of the two coat proteins are sufficiently similar that each is easily converted by mutation to the RNA binding specificity of the other.