Role of cytosol proteins in DNA chain growth and chromatin replication in Friend erythroleukemia cell nuclei.

The influence of cytosol proteins on the replication of DNA and chromatin in isolated nuclei from Friend erythroleukemia cells has been investigated. The overall process has been clearly shown to proceed stepwise. In the absence of cytosol proteins DNA chain growth tends to stop after the addition of approximately 200 nucleotides to the ends of growing chains. In the presence of cytosol proteins these sections grow to approximately 250 nucleotides, and participate in the stepwise extension of the replication process through adjacent nucleosomal sections of the template. Immediately following pulse labeling, the newly replicated DNA resides in a chromatin form which appears to be relatively resistant to digestion by micrococcal nuclease. During a chase interval, the association of the pulse-labeled DNA with nuclear proteins matures to a form which yields lengths of DNA upon digestion with micrococcal nuclease that correspond to mono-, di-, tri- and polynucleosomal units of chromatin. In the absence of cytosol proteins the nuclease resistant state of the labeled DNA tends to predominate and persist. The data support the view that DNA replication in a chromosomal setting proceeds stepwise over successive nucleosomal sections of template made accessible by the interaction of the cytosol proteins at or near the replication fork.