The anatomy of cooperative binding between protamines and DNA

DNA was titrated with protamine under equilibrium conditions while binding was simultaneously followed by electron microscopy and the fluorescamine technique. Positive cooperativity concluded from the binding assays was shown to be correlated with an aligned association of DNA fibres. Subsequent structural modifications, dependent upon the protamine/DNA ratio, suggests 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.