The chromosome of Escherichia coli sediments three different ways in sucrose density gradients, depending upon the concentration of lysozyme used to lyse the cells, as follows: 1. (1) Low-S complex. In the presence of 2-400 μg/ml of egg white lysozyme with Brij-58-Na deoxycholate or Triton X-100, the chromosome sediments at 500-1100 S (low-S complex). When Brij-58 alone is used, some of the DNA sediments as a low-S complex and some sediments rapidly with the membrane fraction. 2. (2) Secondary DNA-membrane complex. In the presence of 800 μg/ml of egg white lysozyme, all the DNA sediments with the membrane fraction at greater than 4000 S regardless of which detergent is used. This DNA-membrane complex is due to the high concentration of lysozyme, for when a pre-formed low-S complex is exposed to a high concentration of lysozyme, it sediments with the membrane fraction. This association can also be seen in the presence of poly(l-lysine), ribonuclease, Mg2+ or spermidine, indicating that lysozyme mediates the DNA-membrane association non-specifically as a polycation. An artificial association between exogenous coalfish DNA and an E. coli membrane fraction is induced by 800 μg/ml of lysozyme. 3. (3) Primary DNA-membrane complex. A concentration of T4 lysozyme as low as 0.01 μg/ml, or 20 lysozyme molecules/chromosome, can lyse cells. This is in contrast to 2 · 106 lysozyme molecules/chromosome corresponding to 800 μg/ml of egg white lysozyme. Under these mild lysis conditions, the chromosome sediments with the membrane fraction, suggesting a primary DNA-membrane association.
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