DNA-INDUCED CONFORMATIONAL-CHANGES IN RECA PROTEIN - EVIDENCE FOR STRUCTURAL HETEROGENEITY AMONG NUCLEOPROTEIN FILAMENTS AND IMPLICATIONS FOR HOMOLOGOUS PAIRING

We have used circular dichroism as a probe to characterize the solutio n conformational changes in RecA protein upon binding to DNA. This app roach revealed that RecA protein acquires significant amounts of alpha -helix upon interaction with DNA. These observations, consistent with the data from crystal structure (Story, R. M., Weber, I., and Steitz, T. (1992) Nature 355, 318-325), support the notion that some basic dom ains including the DNA binding motifs of RecA protein are unstructured and might contribute to the formation of alpha-helix. A comparison of nucleoprotein filaments comprised of RecA protein and a variety of DN A substrates revealed important structural heterogeneity. The most sig nificant difference was observed with poly(dG) . poly(dC) and related polymers, rich in GC sequences, which induced minimal amounts of alpha -helix in ReaA protein. The magnitude of induction of alpha-helix in R ecA protein, which occurred concomitant with the production of ternary complexes, was 2-fold higher with homologous than heterologous duplex DNA. Most importantly, the stimulation of ATP hydrolysis by high salt coincided with that of the induction of alpha-helix in RecA protein. These conformational differences provide a basis for thinking about th e biochemical and structural transitions that RecA protein experiences during the formal steps of presynapsis, recognition, and alignment of homologous sequences.