DNA stores the information for building proteins in the cell, but the message must go through an RNA intermediate. The large multi–subunit complex that reads the DNA sequence and creates mRNA is RNA Polymerase (RNAP), found in every living organism. Bacterial RNAP contains 6 subunits (ββ'ααωδ). The ββ' subunits form several distinct functional channels that accommodate double stranded DNA and the RNA–DNA hybrid as well as the exit channel that guides the growing RNA strand out of the complex and the secondary channel that allows nucleotides to enter the active site. This model focuses on the β' subunit that contains the highly conserved active site sequence and several structures involved in the catalytic mechanism. The bridge helix (BH) and trigger loop (TL) work together as a "gate" to enhance the catalytic action by facilitating nucleotide addition. In the crystal structure of the RNAP elongation complex (EC) without NTP in the active site the TL (β' 1236-1265) is unstructured. In the EC crystal structure with a non-hydrolysable nucleotide (AMPcPP) the TL folds into two anti-parallel helices (trigger helix, TH) that interact with the adjacent BH to create a three helical bundle forming a catalytically active complex. The other structures that are functionally important in the β' subunit are the "lid" (β' 526-539) that cleaves the RNA–DNA directing the newly formed RNA out through the exit channel and the "rudder" (β' 586-601) that helps to stabilize the DNA helix and the RNA–DNA hybrid in the active site channel.
Models in this Collection
RNA polymerase beta subunit
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