Derivatives of human SRP-RNA were constructed by site-directed mutagenesis and tested for their ability to interact with protein SRP19. An RNA missing helix 6 barely interacts with SRP19, while the helix 8-deletion mutant retains much binding capability. A mutant RNA consisting just of helix 6 also binds the protein, but not as well as the unaltered molecule. SRP19 interacts to a full extent with the fourth mutant RNA composed of helices 6, 7, 8 and a portion of helix 5. It is concluded that helix 6-and not helix 8-is the major SRP19 binding site. Helices 7, 8 and portions of helix 5 contribute to the formation of a functional site. These results agree with data suggesting a proximity of helix 6 and the conserved part of SRP-RNA.
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