1. Protamines were preferentially phosphorylated by the six protein kinase fractions isolated from calf thymus nuclear sap, but histones with the exception of H4 also proved to be acceptable substrates. 2. BSA was not a substrate for the first five kinase fractions, but was the best substrate for the seventh fraction, which also exhibited considerable activity with H4 as substrate. 3. An analysis of in vitro phosphorylation experiments with nuclear sap protein kinases reveals a decreased H2b phosphorylation in H1-depleted chromatin relative to "native" chromatin. 4. With fraction V the initial velocity patterns at fixed ATP levels and varying concentrations of histones exhibit cooperativity at lower concentrations and inhibition at higher concentrations, indicating that nuclear sap kinases might play important roles in sensitive regulatory mechanisms of histone phosphorylation in vivo.
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