An expression is derived for the error which arises from the use of the difference Fourier technique.Errors due to the use of inaccurate values of AF and phase are considered in addition to those due to the difference Fourier technique itself. It is shown that a difference Fourier map is able to detect much smaller features of electron density than those revealed by a normal Fourier map with the same phases. The experimental conditions required to obtain difference maps in which the error level is minimized are outlined.
The chemotactic regulator CheY controls the direction of flagellar rotation in Escherichia coli. We have determined the crystal structure of BeF3--activated CheY from E. coli in complex with an N-terminal peptide derived from its target, FliM. The structure reveals that the first seven residues of the peptide pack against the beta4-H4 loop and helix H4 of CheY in an extended conformation, whereas residues 8-15 form two turns of helix and pack against the H4-beta5-H5 face. The peptide binds the only region of CheY that undergoes noticeable conformational change upon activation and would most likely be sandwiched between activated CheY and the remainder of FliM to reverse the direction of flagellar rotation.
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