Question

• 
  (6 pts) Several mutations of the lac operon have been identified.
  • A bacterial mutant called O^C (for constitutive operator) expresses the lac operon even when lactose is present, hence the name constitutive operator. What kind of mutation in the operator sequence could lead to a constitutive operator?
  • Another mutant strain of bacteria called I^s (for super repressor) leads to repression of the lac operon regardless of the presence or absence of lactose. What kind of mutation in the repressor protein could lead to a super repressor?
  • To overcome the effects of I^s and restore normal function to the lac operon, you supply the bacterial cell with wild-type repressor encoded on an extrachromosomal plasmid. The cell now expresses two repressor proteins, wild-type and I^s. Tell why this strategy is unlikely to work
    .

Answer 1

ANSWER )

IN OC MUTANT THERE IS  MUTATION WHICH RESULT IN CHANGES IN DNA SEQUENCE,   IMPAIR ITS BINDING TO THE LAC REPRESSOR, THUS ITS NOT ABLE TO TURN OFF THE LAC OPERON RESULTING IN CONSTITUTIVE EXPRESSION .

IN IS - THE MUTATION IN DNA SEQUENCE OF I GENE LEAD TO FORMATION OF REPRESSOR, AFFECTING THE INDUCER BINDING SITE THUS IT WILL NO LONGER BIND TO INDUCER, RESULTING THAT EVEN IN THE PRESENCE OF LACTOSE REPRESSOR WILL NOT BIND TO IT AND PERMANENTLY BIND TO THE OPERATOR SITE ACTING AS SUPER SUPPRESSOR.

TO OVERCOME THE EFFECT OF IS IF THE WILD TYPE REPRESSOR IS INTRODUCE VIA EXTRACHROMOSOMAL PLASMID THEN ALSO IT WILL NOT RESTORE THE NORMAL FUNCTIONING BECAUSE REPRESSOR WORK THROUGH TETRAMER FORMATION THAT IS FOUR SUBUNIT TOGETHER FORM COMPLEX WHICH WILL BIND TO OPERATOR AND REPRESSOR GENE IS TRANSACTING ELEMENT THUS THERE WILL BE MIXTURE OF BOTH WILD TYPE AND IS REPRESSOR SUBUNIT TO FORM COMPLEX WHICH WILL NOT ABLE TO BIND TO LACTOSE (INDUCER ) AND BIND TO OPERATOR RESULTING ABSENCE OF NORMAL FUNCTIONING LAC OPERON