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Summary Genetics Chapter 14
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Gene regulation in bacteriaGene Enhances efficiency in energy use. Cell does not waste energy making RNAs and proteins it does not need.
regulation Means level of gene expression can vary under different conditions.
Unregulated genes = constitutive, constantly expressed, encode proteins that are continuously necessary
for survival.
Regulated genes = encoded proteins/RNAs only produced when required.
Regulation can occur at any point in the pathway to gene expression, but most commonly at transcription
initiation level -> rate of mRNA production ; increased or decreased
Involves two Repressors: inhibit transcription; negative control
main types of Activators: increase transcription; positive control
regulatory Both bind directly or indirectly to DNA, also have two bindings sites; one for small effector molecules, one
proteins for DNA.
Small effector molecules; bind to regulatory proteins not directly to DNA
Small effector Inducers: increase transcription, gene regulation inducible
molecules Inducer bind activator and cause them to bind to DNA
Inducer bind repressor and prevent them from binding to DNA
Inhibitors: inhibit transcription
Corepressor bind to repressor and cause them to bind to DNA
Bind to activator and prevent them from binding to DNA
Enzyme Particular enzyme appears in the cell only after the cell has been exposed to the enzymes substrate. When
adaptation bacteria is not exposed to particular substance, it does not make the enzymes needed to metabolize this
substance -> E.g. lactose metabolism.
Operon A regulatory unit (group) consisting of a few structural genes under the control of one promoter. Very
common in bacteria: genes arranged together in an operon. However, not all genes in bacteria are in an
operon. In a operon there is 1 promoter.
The promoter of It encodes polycistronic (copy-ORF) mRNA: contains the coding sequence for 2/+ structural proteins.
an operon is
where transcription It allows a bacteria to co-ordinately regulate a group of genes that encode proteins involved in the same
begins. If the process/function -> expression of a gene occurs in a unit.
promoter
sequence is It contains: promoter, operator, structural genes, terminator.
deleted,
transcription will
not take place.
, Lac operon = Two distinct transcriptional units;
inducible 1. The lac operon:
regulation DNA elements/sequences involved in
transcriptional regulation (same promoter
region in Euk.
Promoter: binds RNA polymerase (LacP)
Operator: binds lac repressor protein (lacO) (3
operator sites)
CAPsite: binds Catabolite Activator Protein (CAP)
Structural genes: contain coding sequences for the enzymes
Lac Z gene: encodes B-galactosidase; enzymatically cleaves lactose and lactose analogous in
galactose and glucose. Converts lactose into allolactose (acts as small effector protein)
Lac Y gene: encodes lactose permease; membrane protein required for transport of lactose and
analogues into cytoplasm.
Lac A gene: encodes transacetylase
2. The lacl gene: codes for regulatory protein: lac repressor (homotetramer). Not considered part of
lac operon. Has its own promoter; i promoter. Constitutively expressed at fairly low levels. Only
small amount of protein is needed to repress the lac operon.
1. Inducible negative control mechanism: repressor + inducer
Negative control Involves lac repressor protein -> when bound ot lacO prevents RNA polymerase from transcribing. In
refers to
transcriptional absence of alloctose, lac repressor is bound to lacO. When inducer alloctose binds to lac repressor it
regulation by a inactivates it and prevents binding to DNA, RNA polymerase can start transcribing. Repressor does not
repressor. An
inducer is a small
effector molecule
that causes the
rate of
transcription to
increase. One way
that it does this is
to bind a repressor
and prevent it from
binding the DNA.
Therefore, an
inducible gene that
is under negative
control will be
transcribed when a
repressor and an
inducer is present.
completely inhibit trnascription; very small amount are made.
Proteins involved with lactose utilization are
fairly stable, but wille eventually be degraded.
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