Repressor-RNA complex
The RNA bacteriophage MS2 is a convenient model system for the study of protein-RNA interactions. The MS2 coat protein achieves control of two distinct processes - sequence-specifik RNA encapsidation and repression of replicase translation - by binding to an RNA stem-loop structure of 19 nucleotides containing the initiation codon of the replicase gene. The binding of a coat protein dimer to this hairpin shuts off synthesis of the viral replicase, switching the viral replication cycle to virus assembly rather than continued replication.
The crystal structure of a complex between recombinant MS2 capsids and the 19-nucleotide RNA fragment has been determined to 3.0 A resolution. The structure shows sequence-specific interactions between conserved residues on the protein and RNA bases essential for binding.