Low levels of non-histone chromosomal proteins in trout testis chromatin.

In studies on the regulation of gene expression, there has been considerable interest in the non-histone (NHC) proteins associated with histones and DNA in chromatin [l-5] . Such NHC proteins are reportedly present in large amounts, i.e. 50-l 50% of DNA content by weight [ 1,2] , and display some tissue and species specificity [l-5] , which has implicated them in gene activation and regulation [l-8] . Other NHC proteins would have structural or enzymatic functions in chromatin. However, it is evident that in the isolation of NHC proteins: (i) structural rather than regulatory proteins would likely be detected [3,5,14] ; and (ii) of these putative structural NHC proteins, some may be artifacts of cytoplasmic [9] , membrane [ 10,191 or other [20 3 contamination. For trout testis, the large ratio of nuclear to cytoplasmic volume makes it relatively easy to prepare ‘clean’ chromatin and examine its NHC proteins. We wish to report that NHC proteins are present at less than 5% of DNA content by weight. This result holds even for early testis, where DNA, RNA and protein synthesis, as well as cell division, are still ongoing processes [ 111, and the structure [ 121 and function of chromatin are similar to those of other tissues. The same observation has been made in calf thymus [ 131, another tissue from which chromatin is readily purified.