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Bending of the helical axis

Title		`STRUCTURE OF A TRANSCRIBING T7 RNA POLYMERASE INITIATION COMPLEX`
PDB code		`1QLN (PDB summary)`
NDB code		`PH0002 (NDB atlas)`
Duplex length		`13 base pairs`
Protein		`RNA polymerase bacteriophage T7, Polymerase`

Table 1. Bending parameters determined with the program ARC_FIT.
The curvilinear helical axis is calculated with CURVES, and bending of this axis is analysed with ARC_FIT. This program allows to fit a straight line, a plane, an arc, a kinked line, and a double kinked line to the helical axis. Good fits are indicated by low values of sigma². An arc is fitted if there are at least 7 bp. A kinked line requires at least 8 bp, and the double kinked line requires at least 13 bp. The geometrical parameters are given for all models fitted, but they characterize the shape of the helical axis only if the fit is good. See Help for further explanations.

Model	sigma² / Å²	Geometrical parameters
Plane	0.148
Straight line (x-axis)	0.898
Arc	0.317	Radius of curvature: r = 99 Å
Kinked line	0.405	Kink angle: alpha = 13° Lengths of straight segments: l₁ = 6 bp, l₂ = 7 bp
Double kinked line	0.310	Kink angles: alpha₁ = 13°, alpha₂ = 12° Twist angle: omega = 84° Lengths of straight segments: l₁ = 4 bp, l₂ = 6 bp, l₃ = 3 bp