A second type of constitutive mutant inactivates the operator site and is known as a Lacoc mutation. Lacoc mutations are dominant as revealed in tests of the appropriate merodiploids: -IPTG +工PTG Interpretation oc is constitutⅳve oc Z*/F0+Z+ oc is dominant you might think that on the basis of a dominance test we could tell whether we have a Lacoc or a Lacr mutation. However, life is not so simple, because it is possible to find Lacr mutations that are dominant. Such mutations are known as LacI-d. they are dominant because the repressor protein is a tetramer and laci-d mutant subunits can combine with normal subunits and interfere with their function IPTG IPTG Interpretatio I-d z+ 工 d is constitutive rdz/F卫z◆ I-d is dominant We will now consider a new genetic test that will let us distinguish Lacoc(operator constitutive) from LacI-d(dominant repressor negative) mutations Cis/trans test IPTG +IPTg Interpretation I-O+Z+ I-d Z+/FI+Z I-d is dominant in cis or in trans with z+ Therefore we say it is"trans-acting 工dz/FIz·◆ (trans) oc Z+/F0+Z Oc is dominant only in cis with Z*. Therefore we say it is "cis-acting OcZ/F 0+Z+ rans If a mutation is cis-acting we take this as evidence that the mutation affects a site on DNA like an operator. If a mutation is trans-acting we take this as evidence that the mutation affects a diffusible gene product such as a repressorA second type of constitutive mutant inactivates the operator site and is known as a LacOc mutation. LacOc mutations are dominant as revealed in tests of the appropriate merodiploids: –IPTG +IPTG Interpretation Oc Z+ + + Oc is constitutive Oc Z+/ F’O+Z+ + + Oc is dominant You might think that on the basis of a dominance test we could tell whether we have a LacOc or a LacI– mutation. However, life is not so simple, because it is possible to find LacI– mutations that are dominant. Such mutations are known as LacI-d. They are dominant because the repressor protein is a tetramer and LacI-d mutant subunits can combine with normal subunits and interfere with their function. –IPTG +IPTG Interpretation I-d Z+ + + I-d is constitutive I-dZ+/ F’I+Z+ + + I-d is dominant We will now consider a new genetic test that will let us distinguish LacOc (operator constitutive) from LacI-d (dominant repressor negative) mutations. Cis/trans test –IPTG +IPTG Interpretation I+O+Z+ – + I-d Z+/ F’I+Z– + + I-d is dominant in cis or in trans with Z+; (cis) Therefore we say it is “trans-acting”. I-d Z–/ F’I+Z+ + + (trans) Oc Z+/ F’O+Z– + + Oc is dominant only in cis with Z+; (cis) Therefore we say it is “cis-acting”. Oc Z–/ F’O+Z+ – + (trans) If a mutation is cis-acting we take this as evidence that the mutation affects a site on DNA like an operator. If a mutation is trans-acting we take this as evidence that the mutation affects a diffusible gene product such as a repressor