Left-handed DNA helices

Abstract

The synthetic DNA polymerpoly d(GC) . poly d(GC) has been studied by X-ray diffraction of orientated fibres in which it has either the well known (A or B) randed conformations have been observed with poly d(AC) . poly d(GT) and with poly d(As4T) . poly d(As4T), demonstrating that the novel structure is accessible to any DNA segment with an alternating purine-pyrimidine base sequence.

Raison d'être and structural model for the B‐Z transition of poly d(G‐C)∗poly d(G‐C)

“…However, the A form was trapped in fibers of the Na+ salt up to 92% relative humidity [2,3], and it was monitored by CD spectroscopy if trifluoroethanol was added to an aqueous solution of poly d(G-C) -poly d(G-C) [7]. Under these latter conditions, slow transitions A-Z and B-Z were observed and a fast B-A transition which occurred in about 10 s. An A&Z transition has been described in the ribo series for poly r(G-C) -poly r(G-C)…”

“…The parameters listed are those of Dickerson and colleagues [26,2?f and the sign convention is that adopted at the 1988 Cambridge meeting f28]. CIearly, the CpG step is unique by showing significant& reduced helical twist and propeller and a farge negative slide of -2.2 A in addition to the previously noted extended backbone nonformation.…”

“…poly d(G-C). The A form was trapped in the fiber state [2,3] where it is probably stabilized by crystal packing forces. X-ray fiber diffraction clearly indicates that poly d(G-C) .…”

“…The Z-DNA form is also found in fibers under high-salt conditions [2,3]. NMR data suggest that the B-Z transition does not involve opening or breaking of the G-C Watson-Crick base pairs [6].…”

Antiserum to Z‐DNA

“…However, X-rays studies on crystals of alternating d(CpG)-DNA fragments and on fibers of poly(dG-dC) • poly(dG-dC) have demonstrated a left-handed double helix termed Z-DNA [1][2][3][4]. In solution, poly-(dG-dC) • poly(dG-dC) can adopt the B-or Z-form depending upon the ionic strength of the medium [5][6][7][8][9].…”