[Distribution of large ligands on DNA macromolecules of different GC-content]

The complexes of calf thymus DNA, E-coli DNA, phage T2 and T7 DNA with protamine and poli-lysine (mol. wt. 52 000 and 20 000) were studied by thermal denaturation method. The average lengths of free and ligand-covered regions of DNA were determined from the melting curves by means of the formulae derived in the theoretical works by Akhrem A. A. and oths. [5]. The calculations show that poly-l-lysine (PL20000) and protamine are distributed along macromolecules of all the studied DNAs by clusters containing in the average four ligand molecules when NH2/P less than 0.25. The number of ligands in the clusters is doubled when NH2/P greater than or equal to 0.5 and increased three-sixteen times depending on the types of DNA when NH2/P greater than or equal to 0.75. The molecules of PL52000 are distributed along the DNA macromolecules isolated from each other, every one covering 120 base pairs. The clusters average lengths and their distribution along the DNA macromolecules do not depend directly on GC-content of DNA, but are defined by the concrete pair of the ligand-lattice. Possible cause of cooperative binding of large ligands with DNA is discussed.