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 xy/R Ser to Leu mutatio n at position 41 of the encoded amino acid sequence showing a constitu tive expression phenotype for the xyl operon. When cloned on a multi-c opy plasmid in a B. megaterium strain harbouring a single-copy xylA-la cZ fusion it leads to derepression of beta-galactosidase expression. T hus, it is trans dominant over the endogenous xylR, indicating that Xy l repressor functions as a multimer. This result also supports the ass umption that the mutation is in a putative alpha-helix-turn-alpha-heli x operator binding motif of Xyl repressor.