In MS2 assembly of phage particles results from an interaction between a coat protein dimer and a stem-loop of the RNA genome (the operator hairpin). Amino acid residues Thr45, which is universally conserved among the small RNA phages, and Thr59 are part of the specific RNA binding pocket and interact directly with the RNA; the former through a hydrogen bond, the latter through hydrophobic contacts. The crystal structures of MS2 protein capsids formed by mutants Thr45Ala and Thr59Ser, both with and without the 19 nt wild-type operator hairpin bound, are reported here. The RNA hairpin binds to these mutants in a similar way to its binding to wild-type protein. In a companion paper both mutants are shown to be deficient in RNA binding in an in vivo assay, but in vitro the equilibrium dissociation constant is significantly higher than wild-type for the Thr45Ala mutant. The change in binding affinity of the Thr45Ala mutant is probably a direct consequence of removal of direct hydrogen bonds between the protein and the RNA. The properties of the Thr59Ser mutant are more difficult to explain, but are consistent with a loss of non-polar contact.