Composition of The 7SK SnRNP
An early study indicated that 7SK in cells is associated with a number of proteins and probing of the secondary structure suggested a model for base pairing between different regions of the RNA. A breakthrough in the function of the 7SK snRNP came with the finding that the positive transcription elongation factor P-TEFb was a component of the complex. 7SK associates with and inhibits the cyclin dependent kinase activity of P-TEFb through the action of the RNA binding proteins HEXIM1 or HEXIM2. The gamma phosphate at the 5' end of 7SK is methylated by the methylphosphate capping enzyme MEPCE which is a constitutive component of the 7SK snRNP. A La related protein LARP7 is also found associated with 7SK, presumably in part through its interaction with the 3' end of the RNA. Reduction of either MEPCE or LARP7 by siRNA mediated knockdown leads to destabilization of 7SK in vivo. A subset of 7SK snRNPs lack P-TEFb and HEXIM, but contains hnRNPs instead.
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