The L2 loop peptide of RecA stiffens and restricts base motions of single-stranded DNA similar to the intact protein

The L2 loop in the RecA protein is the catalytic center for DNA strand exch ange, Here we investigate the DMA binding properties of the L2 loop peptide using optical spectroscopy with polarized light. Both fluorescence intensi ty and anisotropy of an etheno-modified poly(dA) increase upon peptide bind ing, indicate that the base motions of single-stranded DNA are restricted i n the complex. In agreement with this conclusion, the peptide-poly(dT) comp lex exhibits a significant linear dichroism signal. The peptide is also fou nd to modify the structure of double-stranded DNA, but does not denature it . It is inferred that strand separation may not be required for the formati on of a joint molecule, (C) 1999 Federation of European Biochemical Societi es.