Identification of histone H1(0) in Physarum polycephalum. Its high level in the plasmodial stage increases in amount and phosphorylation in the sclerotial stage.

An antiserum specific for the globular domain of the bovine very lysine-rich histone subfraction H1(0) cross-reacted with a single protein band in the chromosomal proteins isolated from microplasmodia of the true slime mold Physarum polycephalum. Its amino acid composition was characteristic of a very lysine-rich histone which supported its identification as Physarum H1(0). Unlike Physarum H1(0), which is 50% larger than mammalian H1, Physarum H1(0) migrated very close to bovine H1(0) on sodium dodecyl sulfate gels. In microplasmodia, the ratio of H1(0) to H1 was 0.66, whereas in sclerotia H1(0)/H1 was 1.33. Furthermore, both H1 and H1(0) in sclerotia were highly phosphorylated. The high level of H1(0) in the mitotically active microplasmodia argues against the proposed role of H1(0) just as an inhibitor of DNA replication. More probable is an association of H1(0) with quiescent but transcriptionally competent chromatin which could also include cell cycle genes. Hyperphosphorylation of H1 and H1(0) in sclerotia is probably required to maintain an inactive condensed state which can be reversed by dephosphorylation to allow transcriptionally competent chromatin to become available for expression.