THE LAC OPERON 2) Structural genes à genes that make a specific RNA or protein not involved in the
regulation of the expression of a gene Operons often include: 1) Regulatory sequences like promoters
and operators 3) Regulatory genes à genes that make a specific RNA or protein involved in the
regulation of the expression of a gene Example: Transcription factors Example: hemoglobin or enzymes
that make nutrients
GENE REGULATION IN PROKARYOTES Genes and operons can be under negative control Transcription is
blocked or repressed by a regulatory molecule Genes and operons can be under positive control
Transcription is stimulated or induced by a regulatory molecule This is called a repressible operon This is
called an inducible operon
THE LAC OPERON Metabolism is a general term describing most chemical reactions involved in living
cells It can be divided into catabolism and anabolism Anabolism à synthesizing complex molecules
Catabolism à breaking down large molecules
THE LAC OPERON Lactose is a sugar, consisting of one molecule of glucose and one molecule of
galactose Lactose must be catabolized into glucose and galactose to serve as a carbon source (i.e.
“food”)
THE LAC OPERON Genes for lactose catabolism are induced (expressed at higher levels) when lactose is
present in the environment E. coli prefers glucose, but will use lactose if there is no glucose in its
environment When lactose is high, the lac operon is expressed The purpose is to breakdown the lactose,
so the bacteria can use it for food
THE LAC OPERON The P region is the promoter The lac operon contains a regulatory region that can be
divided into two smaller regions
THE LAC OPERON The lac operon contains a regulatory region that can be divided into two smaller
regions The O region is the operator
THE LAC OPERON LacZ LacY LacA The lac operon also contains three structural genes
THE LAC OPERON Lactose induces the transcription of the structural genes (LacZ, LacY, and LacA) in the
lac operon When lactose levels are high, the lactose needs to be broken down so the lac operon is
turned on When lactose levels are low, the lac operon needs to be turned off
THE LAC OPERON In the absence of lactose, the lac operon is turned off This is done by Repressor
protein encoded by the LacI gene
THE LAC OPERON When there is low/no lactose, the LacI gene express a repressor protein that binds to
the operator sequence This prevents the RNA polymerase from transcribing the lac operon à no
transcription, no expression LacI RNA pol
THE LAC OPERON When lactose levels are high, the repressor protein now binds to the lactose instead of
the operator sequence LacI RNA pol lactose mRNA This allows the RNA polymerase to transcribe the lac
operon
MUTATIONS AND THE LAC OPERON THE LAC OPERON Constitutive mutations à gain of function à
gene/protein is always active (on) or expressed LacI X RNA pol I - a mutation in lacI that prevents the
repressor from binding to the operator This results in the lac operon always being turned on THE LAC OPERON Mutations in the operator region (lac OC) also causes constitutive expression X LacI
RNA pol This results in the lac operon always being turned on
THE LAC OPERON X P- à is a mutation in the promoter sequence that prevents the RNA polymerase from
binding This results in the lac operon always being turned off RNA pol lactose
THE LAC OPERON X Z- or Y- or A- à are mutations in one of the structural genes This results in the lac
operon being regulated normally, but you don’t expression of that specific gene
THE LAC OPERON If it is always on, then you have two possibilities: The repressor (I- ) or the operator
(OC) is mutated If it is always off, then you have two possibilities: The repressor (IS) or the promoter (P- )
is mutated If one gene is off, then that gene is mutated (LacZ- , LacYor LacA- )
THE LAC OPERON 2)Now you need to think about rescuing the lac operon using an F’ plasmid The F’
plasmid is a circular piece of DNA that contains genes that the bacterial cell needs It is NOT a part of the
bacterial chromosome
THE LAC OPERON 2)Now you need to think about rescuing the lac operon using an F’ plasmid The
plasmid can contain a fully functional lac operon Depending on the type of mutation, the plasmid lac
operon can rescue or restore the phenotype back to normal
THE LAC OPERON The plasmid made repressor can bind to the operator of the chromosomal lac operon
LacI This would restore the regulation of the lac operon 1) The repressor (I-
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