Selective interaction of 5'-bromodeoxyuridine substituted DNA with different chromosomal proteins

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Abstract
Chromosomal proteins electively interact with 5''-bromodeoxyuridine (BrdUrd) substituted DNA relative to unsubstituted DNA. The relative affinities of chromosomal proteins for BrdUrd-DNA and unsubstituted DNA [hepatoma tissue culture] were measured by both thermal chromatography on hydroxylapatite and selective retention on nitrocellulose filters. Certain chromosomal proteins have a high affinity for hydroxylapaptide; during thermal chromatography of chromatin, the single-stranded DNA component percolates across a bed of adsorbed proteins as it elutes. The relative affinities of BrdUrd-DNA and normal DNA for chromosomal proteins were measured by chromatographing appropriate mixtures on hydroxylapatite. Under these conditions, the histone components, rather than the nonhistone chromatin proteins, retard the BrdUrd-substituted DNA. The individual histones vary in the degree of their affinity for BrdUrd-DNA in the order H3 > H4 > H2A > H2B > H1. The property that protein-DNA complexes have a preferential affinity for nitrocellulose filters over naked DNA was used to measure the selective binding of BrdUrd-DNA and unsubstituted DNA to both histone and nonhistone chromosomal proteins at low temperatures. The histones selectively retained BrdUrd-DNA on filters in the order H4 > H2A > H3 > H2B > H1. Using this assay, the nonhistones displayed greater selectivity toward BrdUrd-DNA than the histone fraction. BrdUrd-containing DNA probably has a specific affinity for certain chromosomal proteins. The selective interaction of chromosomal proteins with BrdUrd-DNA may be the basis for selective inhibition of cytodifferentiation by the thymidine analog, BrdUrd.