The subunit structure of chromatin: characteristics of nucleohistone and nucleoprotamine from developing trout testis.

Recent work has demonstrated the existence of a subunit structure in chromatin. Hewish and Burgoyne have reported [I] that a large proportion of DNA in isolated rat liver nuclei is digested by an endogenous Ca++ and Mg+’ dependent endonuclease into fragments which are integral multiples of a unit length. Louie has found [2] that DNA from mouse liver nuclei or from the nucleoprotein complex of polyoma virus is also digested by this endonuclease into 200 base pair fragments. An important breakthrough has been provided by No11 [3] who has obtained similar DNA fragments from rat liver nuclei by digestion with commercially available DNase or micrococcal nuclease (E.C. 3.1.4.7). He has also succeeded in separating chromatin from nucleasetreated nuclei into 11.2 S ‘subunit monomers’ (containing chromosomal proteins on 200 base pairs of DNA) and dimers, trimers etc. on sucrose gradients. Trout testis is a good tissue for the study of such chromatin subunits since: (i) large quantities of chromatin relatively free of cytoplasmic contamination are easily prepared; (ii) the subunits represent one approach to studying changes in chromatin structure when the histones are replaced by protamine during sperm development; and (iii) the subunits provide a model substrate for the study of the enzymatic modifications of histones in trout testis. We wish to report that 11 S chromatin subunits can be isolated from trout testis nuclei treated with micrococcal nuclease at early stages (containing nucleohistone) of development, but that trout sperm nuclei