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(Received for publication, January 31, 1997, and in revised form, March 5, 1997)
From the Department of Microbiology and Immunology, Kimmel Cancer
Institute, Thomas Jefferson University, Philadelphia, Pennsylvania
19107
Zinc fingers in transcription factor IIIA
(TFIIIA) contribute differentially to RNA and DNA binding affinity. We
investigated whether the same putative
-helix amino acids in TFIIIA
zinc fingers are essential for both RNA and DNA binding. In published
structures, zinc fingers make DNA base contacts through amino acids
1, +2, +3, and +6 of the recognition helix. Alanine substitution at
these four positions were made in TFIIIA RNA binding zinc fingers,
tz4-7 and DNA binding zinc fingers, tz1-3. Substitution in zinc
fingers 4 or 6 of tz4-7 reduced RNA affinity 77- and 38-fold,
respectively, whereas substitution in zinc fingers 5 or 7 had little
effect. DNA binding affinity of tz1-3 was eliminated by alanine
substitution in any one zinc finger. We determined which amino acids
supported RNA binding by phage display of a library of zinc finger 4 mutants. Lysine at helix position
1 of zinc finger 4 was conserved in all selected tz4-7 fusions. Point mutation of Lys
1
to alanine in zinc finger 4 reduced tz4-7 RNA affinity 30-fold. We
propose that RNA binding by TFIIIA shows similarity to DNA binding in
the use of the recognition helix. Helix positions
1 and +2 may have
particular significance for RNA binding.
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