SMALL ABUNDANT DNA-BINDING PROTEINS FROM THE THERMOACIDOPHILIC ARCHAEON SULFOLOBUS-SHIBATAE CONSTRAIN NEGATIVE DNA SUPERCOILS

Major DNA binding proteins, designated Ssh7, were purified from the th ermoacidophilic archaeon Sulfolobus shibatae. The Ssh7 proteins have a n apparent molecular mass of 6.5 kDa and are similar to the 7-kDa DNA binding proteins from Sulfolobus acidocaldarius and Sulfolobus solfata ricus in N-terminal amino acid sequence. The proteins constitute about 4.8% of the cellular protein. Upon binding to DNA, the Ssh7 proteins constrain negative supercoils. At the tested Ssh7/DNA mass ratios (0 t o 1.65), one negative supercoil was taken up by approximately 20 Ssh7 molecules. Our results, together with the observation that the viral D NA isolated from S. shibatae is relaxed, suggest that regions of free DNA in the S. shibatae genome, if present, are highly positively super coiled.