(i) Regulatory gene in this operon is i gene.
Role of regulatory gene in switching off operon is as follows:
(a) It codes for the repressor protein of the operon, which is synthesised constitutively.
(b) The repressor has affinity for the operator gene. It
binds to operator and prevents the RNA polymerase from transcribing the structural genes.
(ii) When repressor binds to the operator, the operon is switched off and transcription is stopped. So, it is called a negative regulation.
(iii) Lactose is an inducer molecule.
Gene 'z' codes for beta-galactosidase, which is responsible for the hydrolysis of lactose into galactose and glucose, y gene codes for permease. It increases the permeability of the cell to lactose.