Discovery of the operon: Jacob and Monod In study of induction of lactose metabolism, Jacob and Monod isolated phenotypic mutants of two kinds. We now know the molecular basis for these mutant phenotypes. 1. uninducible- I 3 mutant repressor cannot bind inducer, P - mutant promoter (-10/-35 sequences) 2. constitutive- I- lacI mutant (no protein made); O c mutant operator sequence w/lower affinity for repressor, I d mutant repressor can t bind to the operator. They did not have operon sequence or protein structural data, but they could assay the phenotype of each mutant. In hindsight, we can fill in this table to test our understanding of the system, assuming a single O sequence. (In E.coli grown on glycerol, IPTG is used as the inducer b/c it is not metabolized.) Fill in the table below. A " + " indicates that the protein listed would be expressed in the given conditions; a " indicates it would not be expressed. Jacob and Monod had a model for two classes of regulatory factors-cis (DNA elements) and trans (proteins). But without knowledge of the operon structure and the nature of the proteins, how could they distinguish O C from I I 4 ( 5 vs. 4,6 above), or between I 3 and P ( 7 vs. 8 above)? A genetic cis trans test (meridiploids) was used to form a model of the operon. They made partial diploids by introducing the operon on a phage and determined the phenotype of these strains. Fill in the table, using the same symbols as above. In the column labeled "genetic" indicate if the I. P or O mutation is cis or trans and recessive or dominant. For 13 and 14, diagram the lac operon and draw the possible repressor molecules formed..

1. Discovery of the operon: Jacob and Monod In study of induction of lactose metabolism, Jacob
and Monod isolated phenotypic mutants of two kinds. We now know the molecular basis for
these mutant phenotypes. 1. uninducible- I 3 mutant repressor cannot bind inducer, P - mutant
promoter (-10/-35 sequences) 2. constitutive- I- lacI mutant (no protein made); O c mutant
operator sequence w/lower affinity for repressor, I d mutant repressor can t bind to the operator.
They did not have operon sequence or protein structural data, but they could assay the phenotype
of each mutant. In hindsight, we can fill in this table to test our understanding of the system,
assuming a single O sequence. (In E.coli grown on glycerol, IPTG is used as the inducer b/c it is
not metabolized.) Fill in the table below. A " + " indicates that the protein listed would be
expressed in the given conditions; a " indicates it would not be expressed. Jacob and Monod had
a model for two classes of regulatory factors-cis (DNA elements) and trans (proteins). But
without knowledge of the operon structure and the nature of the proteins, how could they
distinguish O C from I I 4 ( 5 vs. 4,6 above), or between I 3 and P ( 7 vs. 8 above)? A genetic cis
trans test (meridiploids) was used to form a model of the operon. They made partial diploids by
introducing the operon on a phage and determined the phenotype of these strains. Fill in the
table, using the same symbols as above. In the column labeled "genetic" indicate if the I. P or O
mutation is cis or trans and recessive or dominant. For 13 and 14, diagram the lac operon and
draw the possible repressor molecules formed.