Abstract
The DNA-bending protein TF1 is the Bacillus subtilis bacteriophage SPO1-encoded homolog of the bacterial HU proteins and the Escherichia coli integration host factor. We recently proposed that TF1, which binds with high affinity (Kd was approximately 3 nM) to preferred sites within the hydroxymethyluracil (hmU)-containing phage genome, identifies its binding sites based on sequence-dependent DNA flexibility. Here, we show that two hmU-A base pair steps coinciding with two previously proposed sites of DNA distortion are critical for complex formation. The affinity of TF1 is reduced 10-fold when both of these hmU-A base pair steps are replaced with A-hmU, G-C, or C-G steps; only modest changes in affinity result when substitutions are made at other base pairs of the TF1 binding site. Replacement of all hmU residues with thymine decreases the affinity of TF1 greatly; remarkably, the high affinity is restored when the two hmU-A base pair steps corresponding to previously suggested sites of distortion are reintroduced into otherwise T-containing DNA. T-DNA constructs with 3-base bulges spaced apart by 9 base pairs of duplex also generate nM affinity of TF1. We suggest that twin hmU-A base pair steps located at the proposed sites of distortion are key to target site selection by TF1 and that recognition is based largely, if not entirely, on sequence-dependent DNA flexibility.
Highlights
The genome of the Bacillus subtilis bacteriophage SPO1 contains 5-hydroxymethyluracil1 in place of thymine, and discrimination between T- and hmU-containing DNA is important during phage multiplication [1, 2]
We recently proposed that TF1, which binds with high affinity (Kd was ϳ3 nM) to preferred sites within the hydroxymethyluracil-containing phage genome, identifies its binding sites based on sequence-dependent DNA flexibility
Our previous conjecture that TF1 principally identifies its preferred binding sites through recognition of sequence-dependent DNA flexure was based on an analysis of affinities for DNA constructs with sets of 4-nt loops [5]
Summary
The genome of the Bacillus subtilis bacteriophage SPO1 contains 5-hydroxymethyluracil (hmU)1 in place of thymine, and discrimination between T- and hmU-containing DNA is important during phage multiplication [1, 2]. We show that two hmU-A base pair steps coinciding with two previously proposed sites of DNA distortion are critical for complex formation.
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