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Electrophoretic comparisons of histones and other acid-extractable chromatin proteins isolated from the livers of growing F344 inbred rats and a heterotic paternal hybrid derived by crossing F344 males with Holtzman females reveal significant heterogeneity among a class of nonhistone components soluble in acid. These variations appear to be age and line specific and show more variation in the inbred than the hybrid. Comparisons of the acid-insoluble nonhistone proteins by sodium dodecylsulfate gel electrophoresis reveal significant quantitative changes in a 28,600 dalton polypeptide present in large quantities at 30, 35, and 50 days of age in the hybrid and at 40 and 45 days only in the inbred. Other minor variations were noted in a class of proteins of 40,000--45,000 molecular weight and in those of very high molecular weight (100,000-200,000). Such variations could be a reflection of or prelude to changes in genetic activity and could ultimately be important in the control of growth patterns of developing heterotic animals.

Electrophoretic profiles of acid-extractable proteins from Holtzman rat liver chromatin display four minor and five major histone bands through certain stages of postweaned development but are qualitatively different from the chromatin protein profiles previously reported during postweaned development for the Fisher 344 rat strain and the F344 X Holtzman heterotic progeny [Tallman, G., et al. (1979). Biochem, Genet. 17:185]. The protein profiles from the heterotic progeny do not reflect and are not combinations of the profiles from the parental strains. Levels of in vitro transcription with Escherichia coli RNA polymerase of total chromatin and of acid-extracted chromatin from Holtzman rat liver tissue fluctuate in an age-specific manner during postweaned development and are higher than previously published levels determined for the Fisher and F344 X H strains during the same developmental period [Tallman, G., et al. (1978). J. Hered. 69:282]. The degrees of stimulation in the transcription assays resulting from the acid treatment vary with the age of the animal but are similar for the maternal Holtzman and hybrid strains. These studies suggest that regulation of heterotic growth may involve dominant, or maternal genetic influences.

DNA-dependent RNA polymerase I from E. coli has been used as probe for determining the capacity of purified chromatin isolated from inbred and hybrid rats during the course of postweaning development to serve as template for RNA synthesis in vitro. An analysis of variance reveals both strain- and age-specific differences in the incorporation of [3H]UTP into RNA. The ability of inbred chromatin to support synthesis exceeds that of the hybrid at every age examined except 50 days, with values in the inbred line approaching those in the hybrid line with increasing age. These data suggest that template characteristics may vary substantially with regard to strain and age and may contribute to differences in heterotic development.

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