THE AMINO-TERMINAL DOMAIN OF THE PROKARYOTIC ENHANCER-BINDING PROTEINXYLR IS A SPECIFIC INTRAMOLECULAR REPRESSOR

The mechanism under which the signal-reception amino terminal portion (A domain) of the prokaryotic enhancer-binding protein XylR controls t he activity of the regulator has been investigated through complementa tion tests in vivo, in which the various protein segments were produce d as independent polypeptides, Separate expression of the A domain rep ressed the otherwise constitutive activity of a truncated derivative o f XylR deleted of its A domain (XylR Delta A). Such inhibition was not released by m-xylene, the natural inducer of the system, Repression c aused by the A domain was specific for XylR because it did not affect activation of the sigma(54) promoter PnifH by a derivative of its cogn ate regulator, NifA, deleted of its own A domain, The A domain was als o unable to repress the activity of a NifA-XylR hybrid protein resulti ng from fusing two-thirds of the central domain of NifA to the carboxy l-terminal third of XylR, which includes its DNA-binding domain. The i nhibitory effect caused by the A domain of XylR on XylR Delta A seems, therefore, to result from specific interactions in trans between the two truncated proteins and not from mere hindering of an activating su rface.