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purine riboswitch
A purine riboswitch is a sequence of ribonucleotides in certain messenger RNA (mRNA) that selectively binds to purine ligands via a natural aptamer domain. This binding causes a conformational change in the mRNA that can affect translation by revealing an expression platform for a downstream gene, or by forming a translation-terminating stem-loop. The ultimate effects of such translational regulation often take action to manage an abundance of the instigating purine, and might produce proteins that facilitate purine metabolism or purine membrane uptake.
==Binding Properties==
Purine riboswitches bind to their purine ligands via interactions at a three-way junction formed by junctional helix P1 and hairpin helicies P2 and P3.〔 Associations that a purine makes when it is within this ''binding pocket'' stabilize the three-way junction, and support the ligand-bound conformation of the mRNA molecule. The purine riboswitch can become saturated at concentrations as low as 5 nM, which reflects the need for gene expression to respond quickly and dynamically to changes in purine concentration.
Despite the relative similarity of the purine-binding aptamer domain across different purine riboswitches, one binding pocket can still discriminate for a single type of purine ligand with high selectivity.〔 Critical to this sensitivity is a single difference in the secondary structure of ribonucleotides: in position 74 of the aptamer domain, it has been found that conversion of a cytosine to a uracil changes an aptamer from being guanine sensitive to being adenine sensitive, and vice versa.〔 Such a conversion is owed to the ability of a nucleotide in position 74 to Watson-Crick base pair with the ligand in the binding pocket, and cytosine and uracil's respective ability to preferentially hydrogen-bond with guanine or adenine.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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