A protein from Ustilago which forms an acid-soluble complex with deoxyribonucleic acid.

A protein which can render DNA largely acid-soluble has been purified 1600-fold from high salt extracts of Ustilago maydis. The activity is unusual in that substrate DNA is not made acid-soluble through hydrolysis to small oligomers. Rather, the protein appears to bind to DNA to form a complex which itself is acid-soluble. The activity of conversion of DNA to an acid-soluble form is cold-labile, but the inactivation by cold is reversible by brief heat treatment. Divalent cations stimulate the activity; phosphate is inhibitory. Optimal activity is observed at pH 6.0 and again at pH 9.0. Nucleoside triphosphates and diphosphates stimulate activity at low protein concentrations but are not hydrolyzed during the course of reaction. The protein behaves anomalously on gel filtration columns and is completely excluded by Sephadex or agarose gels. When analyzed by sedimentation velocity, the protein was found to sediment at 5.3 S, the same rate at which a globular protein of 65,000 daltons would sediment. Dependence of activity upon protein concentrations is sigmoidal. K+ and to a lesser degree NH4+ are partially effective in abolishing the lag in the concentration curve. The protein displays a saturation curve when exposed to increasing DNA concentrations. Such a curve could only result from a non-random or cooperative mode of binding of the protein to DNA. A mutant sensitive to gamma and ultraviolet radiation with an abnormally high level of the protein has been found. Haploid populations of the mutant grow slowly and contain a large proportion (10 to 20%) of inviable cells. Diploids are defective in mitotic allelic recombination and fail to complete meiosis. It is speculated that the protein may be important in the regulation of chromosome condensation.