An operator binding-negative mutation of Xyl repressor from Bacillus subtilis is trans dominant in Bacillus megaterium.

We have selected a Bacillus subtilis 168-borne xylR Ser to Leu mutation at position 41 of the encoded amino acid sequence showing a constitutive expression phenotype for the xyl operon. When cloned on a multi-copy plasmid in a B. megaterium strain harbouring a single-copy xylA-lacZ fusion it leads to derepression of beta-galactosidase expression. Thus, it is trans dominant over the endogenous xylR, indicating that Xyl repressor functions as a multimer. This result also supports the assumption that the mutation is in a putative alpha-helix-turn-alpha-helix operator binding motif of Xyl repressor.