Secondary structure switching in Cro protein evolution

We report the solution structure of the Cro protein from bacteriophage P22. Comparisons of its sequence and structure to those of λ Cro strongly suggest an α-to-β secondary structure switching event during Cro evolution. The folds of P22 Cro and λ Cro share a three α helix fragment comprising the N-terminal half of the domain. However, P22 Cro's C terminus folds as two helices, while λ Cro's folds as a β hairpin. The all-α fold found for P22 Cro appears to be ancestral, since it also occurs in cI proteins, which are anciently duplicated paralogues of Cro. PSI-BLAST and transitive homology analyses strongly suggest that the sequences of P22 Cro and λ Cro are globally homologous despite encoding different folds. The α+β fold of λ Cro therefore likely evolved from its all-α ancestor by homologous secondary structure switching, rather than by nonhomologous replacement of both sequence and structure.