NARL DIMERIZATION - SUGGESTIVE EVIDENCE FROM A NEW CRYSTAL FORM

The structure of the Escherichia coli response regulator NarL has been solved in a new, monoclinic space group, and compared with the earlie r orthorhombic crystal structure. Because the monoclinic crystal has t wo independent NarL molecules per asymmetric unit, we now have three c ompletely independent snapshots of the NarL molecule: two from the mon oclinic form and one from the orthorhombic. Comparison of these three structures shows the following: (a) The pairing of N and C domains of the NarL molecule proposed from the earlier analysis is in fact correc t, although the polypeptide chain connecting domains was, and remains, disordered and not completely visible. The new structure exhibits ide ntical relative orientation of N and C domains, and supplies some of t he missing residues, leaving a gap of only seven amino acids, (b) Exam ination of corresponding features in the three independent NarL molecu les shows that deformations in structure produced by crystal packing a re negligible. (c) The ''telephone receiver'' model of NarL activation is confirmed, The N domain of NarL blocks the binding of DNA to the C domain that would be expected from the helix-turn-helix structure of the C domain. Hence, binding can only occur after significant displace ment of N and C domains. (d) NarL monomers have a strong tendency towa rd dimerization involving contacts between helixes al in the two monom ers, and this may have mechanistic significance in DNA binding. Analog ous involvement of helix alpha 1 in intermolecular contacts is also fo und in UhpA and in the CheY/CheZ complex.