= RNA polymerase bridge helix
{c}
The <RNA polymerase bridge helix> is an <alpha helix> spanning the <active site> region of <RNA polymerase>. In <RNA polymerase II>, it lies near the template <DNA> and growing <RNA>, where conformational changes can alter the space available for template positioning during <transcription (biology)>.<Structural basis of transcription inhibition by alpha-amanitin and implications for RNA polymerase II translocation by Florian Brueckner and Patrick Cramer>[2008, abstract: bridge helix and template-base positions]{ref}
Researchers soaked <transcription elongation complex> crystals with <alpha-amanitin>. The trapped structure showed the entering <DNA> base above a shifted <RNA polymerase bridge helix>, with the usual templating position obstructed. The authors interpreted the arrangement as a possible intermediate in movement along <DNA>.<Structural basis of transcription inhibition by alpha-amanitin and implications for RNA polymerase II translocation by Florian Brueckner and Patrick Cramer>[Abstract: inhibitor soaking and proposed translocation intermediate]{ref}
The useful connection is between local <protein> geometry and which <DNA> base can be presented to the catalytic machinery. Successive addition cycles require that relationship to change while sequence order remains intact. A trapped conformation is evidence about an accessible arrangement; establishing its timing and contribution to normal movement also requires kinetic measurements. The helix is part of the coupled <enzyme>, rather than an independently operating motor.
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