The DNA component of cytochrome b₂: II. The specificity of its association with the enzyme and its origin from high molecular weight DNA

1. 1. The specificity of the association between the DNA component and the enzyme of crystalline Type I yeast cytochrome b₂ (l-lactate:cytochrome c oxidoreductase, EC 1.1.2.3) has been investigated by testing the ability of various nucleic acid preparations to form crystalline complexes with DNA-free (Type II) enzyme. 2. 2. It has been found that only double-stranded DNA molecules with a molecular weight of roughly 2 × 10⁵ will produce the square plate crystals that are characteristic of normal preparations of Type I cytochrome b₂. High molecular weight DNA and single-stranded DNA, either native or denatured, produced either amorphous precipitates or various semi-crystalline forms. These effects were independent of the base composition of the DNA samples used. 3. 3. The ability of b₂-DNA to anneal extensively with all samples of labelled yeast RNA collected after centrifugation on a sucrose density gradient, even with RNA up to 10 times its own size, has been taken as proof that b₂-DNA is a breakdown product of higher molecular weight yeast DNA. This conclusion has been supported to the preparation of DNA-free enzyme from fresh, freeze-dried yeast under conditions that would be expected to minimize DNA breakdown. 4. 4. The ability of the enzyme to select nucleic acids satisfying certain criteria of size and secondary structure has been considered as a model for the assembly of virus particles from coat protein and nucleic acid.