Comparison of the high mobility group proteins and their chromatin distribution in trout testis and liver

The trout testis contains two major high mobility group (HMG) proteins HMG-T and H6 which, although related to the four mammalian HMGs, exhibit distinct variation as evidenced by differences in electrophoretic mobility and amino acid sequence. Previous work using various endonucleases as probes has shown that HMG-T and H6 are located at specific sites in the testis chromatin. The differentiation of testis cells during spermatogenesis is characterized by a unique transition from a histone-packaged genome to one bound by a class of small molecular weight, highly basic proteins, the protamines. Questions arise as to whether any of the HMG variability may be unique to the process of spermatogenesis and whether the histone-protamine transition occurring in most testis cells affects the HMG protein distribution and/or the specificity of the probe. In an attempt to answer these questions, the distribution of the HMG proteins in the chromatin of trout liver, a tissue lacking protamine, has been studied and comparisons made with testis. Liver HMGs exhibit the same electrophoretic characteristics as the testis HMGs indicating that the variability when compared to mammalian HMGs is primarily phylogenetic in origin rather than tissue-specific. Furthermore, micrococcal nuclease digestion of liver nuclei and its effect on the subsequent HMG protein distribution during chromatin fractionation yields a pattern very similar to that for testis, suggesting that the interaction of the HMGs with the remaining testis nucleohistone is not significantly altered by the ongoing transition to nucleoprotamine. Finally, the HMGs represent an unusually high proportion of the total testis non-histone protein population; the implications of this are discussed.