Chemical composition of nucleosomes among domains of calf thymus chromatin differing in micrococcal nuclease accessibility and solubility properties.

Calf thymus chromatin was fractionated by the Sanders' procedure ((1978) J. Cell Biol. 79, 97-109). this procedure involves sequential elutions of micrococcal nuclease-digested nuclei with buffers of increasing ionic strength. Through the use of the nuclei nick translation technique of Levitt et al. (Levitt, A., Axel, R., and Cedar, H. (1979) Dev. Biol. 69, 496-505) which specifically labels the transcriptionally competent regions of the chromosome, the lowest salt eluted, micrococcal nuclease-sensitive chromatin fraction, was found to be enriched in transcriptionally competent chromatin. This chromatin fraction contained approximately equimolar amounts of the core histones and low amounts of histone H1. In addition, this fraction was enriched both in the acetylated species of histone H4 and in the high mobility group (HMG) proteins 14 and 17, but it was depleted in 5-methylcytosine. As the ionic strength of the elution buffers increased, chromatin fractions from less micrococcal nuclease-sensitive chromatin domains were eluted. The nuclease-insensitive fractions were enriched in the unacetylated species of histone H4, 5-methylcytosine, and histone H1. Although these fractions had a smaller proportion of nucleosomes containing HMG-14 and HMG-17, they contained about 50% of the total HMG-14 and HMG-17 population.