Translational repression and specific RNA binding by the coat protein of the Pseudomonas phage PP7

PP7 is a single-strand RNA bacteriophage of Pseudomonas aeroginosa and a distant relative to coliphages like MS2 and Qbeta. Here we show that PP7 coat protein is a specific RNA-binding protein, capable of repressing the translation of sequences fused to the translation initiation region of PP7 replicase. Its RNA binding activity is specific since it represses the translational operator of PP7, but does not repress the operators of the MS2 or Qbeta phages. Conditions for the purification of coat protein and for the reconstitution of its RNA binding activity from disaggregated virus-like particles were established. Its dissociation constant for PP7 operator RNA in vitro was determined to be about 1 nm. Using a genetic system in which coat protein represses translation of a replicase-beta-galactosidase fusion protein, amino acid residues important for binding of PP7 RNA were identified.