Subsets of the four RNA binding domains (RBD 1 to 4) in theXenopuspoly-adenylate binding protein (PABP) have distinct affinities and specificities for RNA. RBDs 1 plus 2 exhibit RNA affinity and selectivity equal to the wild-type (WT) protein. RBDs 3 plus 4 have distinct selectivity and about ten-fold reduced affinity for A₂₃, and the isolated RBDs 2 or 3 or 4 exhibit about 100-fold reduced affinity for A₂₃in comparison to WT. For the full-length protein, independent RNA contacts have been mapped by UV crosslinking with RBDs 1/2 and RBDs 3/4. The carboxy-terminal, non-RBD portion of the protein does not contribute to RNA affinity or selectivity, but confers homodimerization activity on PABP. RBDs 3 and 4 cooperate with the C terminus to gain poly(A) organizing activity, i.e. the ability to form an RNP with multiple, regularly spaced copies of PABP on a poly(A) substrate.