Purification and conformation of ribosomal protein L25 from E. coli ribosome

Ribosomal protein L25 from the large subunit of E. coli ribosomes has been purified using a new procedure involving a 2M LiCl extraction followed by phosphocellulose chromatography in 6 M urea elution buffer. The conformation of purified L25 was studied employing circular dichroism and ultraviolet absorption spectroscopy in reconstitution buffer. The analysis of the far u.v. circular dichroism spectrum of L25 indicates L25 contains approximately 16% alpha-helix and approximately 19% beta-structure. The conformation of L25 was also studied using the predictive methods of Chou & Fasman and Maxfield & Scheraga. Both of these methods predict approximately three times the percent alpha-helix present in L25 as compared with that determined from the analysis of the circular dichroism spectrum. A structure for L25 is predicted which contains two positively charged binding domains and is consistent with published binding data on the interaction of 5S RNA and L25. The large difference in the % alpha-helix as determined from the analysis of the circular dichroism spectrum and the predictive techniques is suggested to result from the denaturing effects of 6 M urea used in the preparation of ribosomal proteins.