./. x01.
Hid.
(1978) 119. 353-359
LETTERTO
THEEDITOR
An Explanation for Rotor Speed Effects Observed during Sedimentation of Large Folded DNA Molecules
Thch sedimentation coefficient, of the isolated nucleoid of Escherichia coli varies in dif%rtnt centrifugal fields (Hecht’ et al.. 1977). Tn a manner qualitatively analogous t,o tllat of large unfolded DNA molecules. the sedimentation coefficient is maximal at low rot,or speeds and is reduced at higher rotor speeds (Rubenstein & Leighton, 1971 : Ormerod & Lehmann, 1971; Kavenoff, 1972: Levin & Hutchinson, 1973; Schutnaker & Zimm, 1973). These changes in the isolated nucleoid are reversible. The phr~tlomenon has been explained for linear unfolded DNA molecules (Zimm, 1974). The c.onfornlatioll of the linear DNA alters during sedimentation because the ends of the polymer experience a great’er frictional fa.ctor than the middle. The resulting mor(’ cixtendeti conformation thereby has an increastbd overall fractional factor. The changt+ during sedimentation of folded DNA molecules. such a< the i.
Hid.
(1978) 119. 353-359
LETTERTO
THEEDITOR
An Explanation for Rotor Speed Effects Observed during Sedimentation of Large Folded DNA Molecules
Thch sedimentation coefficient, of the isolated nucleoid of Escherichia coli varies in dif%rtnt centrifugal fields (Hecht’ et al.. 1977). Tn a manner qualitatively analogous t,o tllat of large unfolded DNA molecules. the sedimentation coefficient is maximal at low rot,or speeds and is reduced at higher rotor speeds (Rubenstein & Leighton, 1971 : Ormerod & Lehmann, 1971; Kavenoff, 1972: Levin & Hutchinson, 1973; Schutnaker & Zimm, 1973). These changes in the isolated nucleoid are reversible. The phr~tlomenon has been explained for linear unfolded DNA molecules (Zimm, 1974). The c.onfornlatioll of the linear DNA alters during sedimentation because the ends of the polymer experience a great’er frictional fa.ctor than the middle. The resulting mor(’ cixtendeti conformation thereby has an increastbd overall fractional factor. The changt+ during sedimentation of folded DNA molecules. such a< the i.
