Nonrandom distribution of chromosomal proteins during cell replication.

The distribution of chromatin-associated proteins in replicating Chinese Hamster ovary cells has been examined using the method described by Taichman and Freedlender (Taichman, L., and Freedlender, E.F. (1976), Biochemistry 15, 447). Cells are grown for several generations in [14C]lysine and thymidine, and then for one generation in the presence of [3H]lysine and 5-bromodeoxyuridine (BrUdRib) and a further generation in cold amino acid and BrUdRib. This protocol produces equal amounts of unifilarly (heavy-light) and bifilarly (heavy-heavy) substituted DNA. Chromatin containing the two types of DNA are separated by sucrose-gradient centrifugation after ultraviolet irradiation. The results indicate that some of the chromatin proteins can segregate with the DNA strand synthesized in the same generation when the cells subsequently replicate. Using chromatin with a protein to DNA ratio of 2.6, in different experiments, 5-22% of the chromatin proteins were estimated to segregate with the appropriate DNA strand, while the remaining proteins were randomly distributed to daughter chromatin. The segregating proteins have not been specifically identified but they migrate in sodium dodecyl sulfate gel electrophoresis in the region where the four smaller histones migrate.