Histone gene expression and chromatin structure in mammalian cell hybrids.

Abstract

DNA isolated from mammalian cell nuclei reveals discrete size patterns when partially digested with micrococcal nuclease. The DNA repeat lengths from different tissues within a species or from different species may vary. These differences have been attributed to the presence of different species of histone H1. To examine the nature of regulation of DNA repeat lengths and their possible relationship to histone H1, several mouse and human cell lines that differ in their DNA repeat lengths were selected and the cell lines themselves and their cell hybrids were examined. (Five mouse .times. mouse and 24 mouse .times. human hybrid cell lines) were analyzed. All the interspecific hybrids exhibited the repeat pattern characteristic of the murine parent. The mouse intraspecific hybrids had a repeat pattern of only 1 of the parents. The partial human chromosome complements retained in the hybrids assume the repeat lenghts exhibited by the mouse cells. Because H1 histones are implicated in the determination of DNA repeat lengths, the regulation of H1 histone expression in these cell hybrids was also investigated. Purified H1 histones were radioactively labeled in vitro, and individual subfractions were subjected to proteolysis followed by gel electrophoresis. The resulting partial peptide maps of H1 histone subfractions A and B were distinguishable from one another and from different cell lines. In the mouse .times. human hybrids analyzed, only the mouse H1 histones were detected. These observations were extended to H2b by analysis of the hybrid cell histone by Triton-acid-urea gels. Neither the DNA repeat length nor histone expression is affected by the presence of any specific human chromosome. The fact that human genes are expressed in these hybrids suggests that the H1 histones of 1 species is able to interact with the chromatin of another species in a biologically functional conformation. Analysis of the intraspecific PG19 .times. B82 (mouse melanoma .times. mouse neoplastic glioma) hybrids reveals the presence of H1 histone subfractions of the B82 mouse cells. Because these hybrids exhibit the nucleosome repeat length only of the PG19 cells, it appears that if histone H1 plays a role in determining the repeat length it does so in consort with other nonhistone chromosomal proteins.