Proteins from the prokaryotic nucleoid

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Abstract
Escherichia coli DNA‐binding proteins NS1, NS2 and NS (NS1 + NS2) react with the protein‐protein bifunctinol cross‐linking reagents dimethylsuberimidate and dimethyladipimidate to yield oligomers up to hexamers. The former reagent, with the longer arm, is more efficient than the other shorter one. Both one‐ and two‐dimensional gel electrophorses show that the cross‐linked trimers are homogeneous, while the dimers appear heterogeneous, suggesting that at least two types of dimers but geometrically equivalent trimers are formed. In the presence of DNA, the cross‐linking reaction with either reagent yields fewer dimers and more of the larger products.The yield of cross‐linked products of various sizes was determined for NS1, NS2 and NS as a function of the protein concentration (0.03–3000 μM), From the results obtained in these experiments, we derived a model of quaternary structure in which dimers and tetramers are predominant in very dilute solutions of the proteins. Above a critical concentration (10–50 μM), interactions among tetramers become increasingly important, yielding octamers and perhaps larger products. Our data do not support a recently proposed model in which the DNA is packaged around a protein disc consisting of 8–10 NS dimers.