INCREASED PHOSPHORYLATION OF HISTONE H1 IN MOUSE FIBROBLASTS TRANSFORMED WITH ONCOGENES OR CONSTITUTIVELY ACTIVE MITOGEN-ACTIVATED PROTEIN-KINASE KINASE

We compared the nucleosomal organization, histone H1 subtypes, and his tone H1 phosphorylated isoforms of ras-transformed and parental 10T1/2 mouse fibroblasts. In agreement with previous studies, we found that ras-transformed mouse fibroblasts have a less condensed chromatin stru cture than normal fibroblasts. ras-transformed and parental 10T1/2 cel ls had similar amounts of H1 subtypes, proteins that have a key role i n the compaction of chromatin. However, labeling studies with P-32 and Western blot experiments with an antiphosphorylated H1 antibody show that interphase ras-transformed cells have higher levels of phosphoryl ated H1 isoforms than parental cells. G(1)/S phase-arrested ras-transf ormed cells had higher amounts of phosphorylated H1 than G(1)/S phase- arrested parental cells. Mouse fibroblasts transformed with fes, mos, raf, myc, or constitutively active mitogen-activated protein (MAP) kin ase kinase had increased levels of phosphorylated H1. These observatio ns suggest that increased phosphorylation of H1 is one of the conseque nces of the persistent activation of the mitogen-activated protein kin ase signal transduction pathway. Indirect immunofluorescent studies sh ow that phosphorylated H1b is localized in centers of RNA splicing in the nucleus, suggesting that this modified H1 subtype is complexed tot ranscriptionally active chromatin.