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The conformation of Escherichia coli ribosomal protein L7/L12 in solution has been studied using spectroscopic and hydrodynamic methods. Circular dichroism studies in the near-ultraviolet region reveal two bands at 262 and 268 nm originating from the tertiary conformational environment of the phenylalanyl residues. Additional characterization of the phenylalanine environment includes an intrinsic fluorescence emission spectrum arising from the phenylalanine fluorophores. Computer analysis of the far-ultraviolet circular dichroism spectrum suggests that L7/L12 contains as much as approximately 76% alpha helix. Hydrodynamic properties of L7/L12, measured with the purpose of providing relevant shape information, include the frictional coefficient ratio (1.84 +/- 0.03) and intrinsic viscosity (28 +/- 0.4 mL/g). The experimentally determined frictional coefficient (6.15 +/- 0.15 X 10(-8) has been compared with theoretical calculations of the same value employing two independent methods and assuming various dimensions for the L7/L12 dimer. Combining the experimental results from this work with those available from the literature, and using conformation predictive methods of Chou & Fasman [P. Y. Chou & G. D. Fasman (1974) Biochemistry 13, 211-222, 222-245] and of Maxfield & Scheraga (F. R. Maxfield & H. A. Scheraga (1976) Biochemistry 15, 5138-5153), several possible molecular models of the L7/L12 dimer have been constructed and critically examined. A model which is consistent with all of the available data is proposed.

The effects of pH, temperature, and guanidinium chloride on the conformation of ribosomal protein L7/L12 have been investigated in order to understand the stability of this protein dimer. The results indicate that many of the molecular forces stabilizing the conformation of the dimer are disrupted at low pH or high temperature. These acid- and thermal-denatured states, however, still retain considerable secondary structure. Approximately half of the alpha-helical content present in the native protein remains intact at pH below 2 and at temperatures above 90 degrees C. Further denaturation of the acid-denatured protein by 6 M guanidinium chloride results in a state which still contains approximately 20% alpha helix. Similar amounts of residual conformation remain when the native L7/L12 dimer is denatured with guanidinium chloride. Thermodynamic analysis of the conformational transitions studied indicates that none is compatible with a simple two-state process. The complexity of these denaturation data and the structural characterizations of the various denatured states are consistent with the possible existence of structural domains in the protein molecule possessing different conformational stabilities.