Two different oligomeric states of the RuvB branch migration motor proteinas revealed by electron microscopy

In prokaryotes, the RuvA, B, and C proteins play major roles at the late st age of DNA homologous recombination, where RuvB complexed with RuvA acts as an ATP-dependent motor for branch migration. The oligomeric structures of negatively stained and frozen hydrated RuvB from Thermus thermophilus HB8 w ere investigated by electron microscopy. RuvB oligomers free of DNA formed a ring structure of about 14 nm in diameter. The averaged top view image cl early indicated a sevenfold symmetry, suggesting that it exists as a heptam er. The RuvB oligomers complexed with duplex DNA formed a smaller ring of a bout 13 nm in diameter. The averaged top view images represented a sixfold symmetry. This difference in oligomerization indicates that the oligomeric structure of RuvB may convert from a heptamer to a hexamer upon DNA binding . In addition, this finding provides the lesson that great care should be t aken in investigating the subunit organizations of DNA binding proteins, be cause their oligomeric states are more sensitive to DNA interactions than e xpected. (C) 2000 Academic Press.