1 99311 nterestingy, the effect of betaine may have been just the opposte, that is, to enhance the difference between the forces requ~red to Induce the trans~t~on in AT-arid CG-r~ch regions 24 The f~tted Kuhn segment length of 15 A rnpes a persstence length of 7.5 A for ssDNA Ths vdue s about half that estrnated by E K Achter end G. Fesenfed iB~ooolyt-ies 10. 1625 (1 971)] for apurineted ssDNA by us~ng g h t scattering ancl sedilnentat~on I: was assumed n that study that 1 M NaCl s a theta solvent for ssDNA. In the present study, the contrectie force on e ssDNA molecule n 1 M NaCl. extrepoated to zero extenson, was about 5 pN (see Fig. 6, nset blue). Ths force offset probably Indicates secondat? st1 ucture forlnation or condensailon with~n the --~olecule If such structure forlned n the sednentation studes, then an erroneously large value for the r~gidty of ssDNA end for RNA could hzve been obtzned 25 In the presence of adenosne trphosphate (ATP) or ATPiylS. RecA ~lndergoes an aloster~c change n t o a high-affnty form that bnds dsDNA cooperatvey n a stocholnetrc rato of 1 RecN3 bp of dsDNA to form a r~ght-handed h e c a falnent [S C \!Vest, An!iii Rev B I O C~J~~ 61. 603 (1 992)] There are six RecA molec~~les and 18 6 bp;turn of the DNA molecule that s overstretched by a factor of 1 5 t~mes its B-form contour length 26. A Klug and F H C Cr~ck [h'atatlii -e 255. 530 (1975)l have suqgtisted that formaton of a few highly bent regons or "knks" n DNA mght be energetcaly fawarable relative to srnooth bendng o. er a longer DNA length. The argument requres that after the ensung of a o c a z e d knk n the DNA molec~lle, the energy re3,u red to bend ;he DNA further by an angle 8 at that locat~on, be smaller than the energy needed to bend the DNA by the same angle before the ensuing of the knk Ths s ndeed obsetved n macroscopic eastc medawhen the deformaton goes beyond the eastc nto the "plastc" regme (a plastc straw s a good example) 27 Carboxyate-polystyrene beads (3 56 ILm n dameter. CV = 2.7'0, Spherotech) were covelently coated vlith streptav~d~n uslng l-ethyl-3-(3-d1methylam1nopropy) carbodmde (EDAC) Each molecule was puled both r~ght and left from the p~pette to determ n e the pont of attachment of the molecule on the ppette bead. Because the o p t c a y trapped bead can rotate freely, but the ppette-trapped bead cannot. Ex can be deterlnned n abso ute un~ts jmlcrometers), In each F-Ex cuwe, data representng the folowng four processes s ~~~p e r i m p o s e d : extendng the molecule to right of the ppette, then relaxing t froln the right, extendng t leftward, then relaxng t from the left. Each data point was taken after a -0.5 ILm change n extension and a 2-s waitn g perod. The force sgna was then averaged for an add~t~ona 2 s and recorded A complete r~ghr-left stretch cycle took about 10 m n . 28 A video showing actual bead-DNA-bead assembly IFlg. 1 B) can be viewed on the Word W!de Web at 29. D W! . Ussey. R \VJ Hoepfner, R R Sinden Met!i-0dS E~z J /~o / 21 2. 262 (1 992) 30.The nat...

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