Protein D1 preferentially binds A + T-rich DNA in vitro and is a component of Drosophila melanogaster nucleosomes containing A + T-rich satellite DNA.

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Abstract
Previous work showed that D1, a 50-kilodalton chromosomal protein of D. melanogaster, is specifically associated with isolated nucleosomes that contain a complex A + T-rich satellite DNA with buoyant density of 1.688 g/ml. D1 is also a component of nucleosomes containing a simple-sequence, pure A + T satellite DNA, buoyant density 1.672 g/ml. Using a modification of a protein blotting technique in which proteins are not exposed to dodecyl sulfate denturation, D1 preferentially binds to A + T-rich double-stranded DNA in vitro, and it is apparently the only abundant nuclear protein in cultured D. melanogaster cells that possesses this property. Synthetic poly[d(A-T)].cntdot.poly[d(A-T)] and poly(dA).cntdot.poly(dT) duplexes effectively compete in vitro with A + T-rich D melanogaster satellite DNA for binding to D1, whereas total Escherichia coli DNA is an extremely poor competitor. D1 is a specific component of A + T-rich, tandemly repeated, heterochromatic regions, which constitute up to 15-20% of the total D. melanogaster genome. Possible functions of D1 protein include compaction of A + T-rich heterochromatin and participation in microtubule-centromere interactions in mitosis. D1 may prevent nonspecific binding to A +T-rich satellite DNA of other nuclear proteins that have a preference for AT-DNA, such as RNA polymerase or regulatory proteins, and may also participate in the higher-order chromatin organization outside tendemly repetitive regions by binding to nonrandomly positioned stretches of A + T-rich DNA.