Maturation of nucleosomal and nonnucleosomal components of nascent chromatin: differential requirements for concurrent protein synthesis.

The DNA of newly replicated chromatin is comprised of two components, distinguishable by their solubility characteristics and requirements for maturation. One of these components possesses core histones, typical nucleosomal structure, a nuclease-resistant core containing 146 base pairs (bp) of new DNA, and all the nucleosomal species found in bulk chromatin (due to bound histone H1 and high mobility group proteins). In addition, this class of nascent chromatin exhibits a shortened repeat length of approximately 165 bp, as opposed to the 288-bp repeat of bulk chromatin. Within 10 min of DNA synthesis, the spacing of mature chromatin is established; the spacing maturation can occur in the absence of protein synthesis. The second class of nascent DNA is distinguished from the nucleosomal component by its insolubility, lack of discernible nucleosomal organization, and dependence on protein synthesis to attain typical subunit structure. This unassembled component is not free DNA, as demonstrated by its intermediate resistance to nucleolytic degradation. The structural properties and maturation requirements of this material suggest that it is the site of de novo nucleosome assembly.