Small acid-soluble spore proteins (SASPs) appear 3-4 hr after the onset of sporulation in Gram-positive bacteria and constitute up to 20% of the protein of mature spores. Previous studies using Bacillus subtilis deletion mutants lacking SASP-alpha and -beta have shown that such mutations abolish the elevated resistance of spores to UV radiation. Analyses using circular dichroism and Fourier-transform infrared spectroscopy now demonstrate that binding alpha/beta-type SASPs to DNA in vitro causes a structural change in DNA, from the B to the A conformation. This may provide the basis whereby alpha/beta-type SASPs confer increased spore UV resistance in vivo--by changing spore DNA conformation, they alter DNA photochemistry such that UV irradiation produces spore photoproduct instead of the more lethal cyclobutane-type thymine dimers.