The Anatomy of Cooperative Binding Between Protamines and DNA

Abstract

DNA [calf thymus] was titrated with protamine [herring sperm] under equilibrium conditions while binding was simultaneously followed by EM and the fluorescamine technique. Positive cooperativity concluded from the binding assays was correlated with an aligned association of DNA fibers. Subsequent structural modifications, depending on the protamine/DNA ratio, suggest a mechanism for DNA compaction by protamines. In the framework of this model, protamine phosphorylation appears to promote formation of DNA interstrand links yielding a very pronounced filigree structure.