Functioning of structural genes is controlled by

Jacob and Monad studied lactose metabolism in E. coli and proposed operon concept, which is applicable for

$A$ : Operator gene is functional when it is not blocked by repressor.
$R$ : Regulator gene produces active protein only which acts on operon system in $E$.coli

E.coli cells with a mutated $Z$ gene of the lac operon cannot grow in medium containing only lactose as the source energy because

In lac operon system lac gene -$I $ codes for

In Escherichia coli, lac operon is induced by