Why does genetic regulation occur in bacteria?
= environment often changes and genetic regulation is focused on adapting to this change
- genes that are not required generally are not expressed unless environmental change makes expression useful
At what levels can gene regulation occur in bacteria and eukaryotes?
- alteration of structure: compactness of DNA (if compact, difficult to transcribe)
- transcription: gene transcribed to make pre-mRNA, amounts and types of pre-mRNA can be regualted
- esp. in eukaryotes processing of pre-mRNA
- whether RNA is translated (regulated to translate later, or right away and quantity of translation)
- protein might need post-transalational modification and this can be regulated to keep protein active or inactive
At which level is the lac operon in E coli regulated?
- primarily at level of transcription
What are constitutively expressed genes?
= genes that specify products like tRNAs, rRNAs, ribosomal proteins, RNA polymerase subunits, enzymes catalyzing housekeeping functions etc
- these are essential components of almost all living cells and need to be continuously (=constitutively) expressed
What are inducible and repressible genes?
= gene products that are needed for cell growth only under certain env. conditions and regulatory mechanisms that allow synthesis of these gene products only when needed
- turned on = induced, shut off = repressed
Key facts about induction of genes for lactose utilization
- glucose is preferred carbon source for bacteria
- lactose used if nothing else available and need to induce enzymes involved in lactose utilization: happens when lactose present and glucose absent
- induction at level of transcription: alters rate of lactose-metabolizing enzyme synthesis (before: low levels, when glucose absent, lactose present: levels go up)
- enzymes involved in catabolic (breakdown) pathways are often inducible
Repression of genes for Trp biosynthesis
- genes are expressed in absence of trp and turned off when trp is available
- repression occurs at level of transcription of trp biosynthetic genes
- enzymes involved in anabolic (synthesis) pathways are often repressible
- so, if trp present, repress enzymes to synthesize it
What is a bacterial operon?
= single transcriptional unit that includes a series of structural genes, a promoter, and an operator = group of structural genes plus sequences that control transcription
- special regulator protein helps control expression of structural genes
How are most structural genes w related functions organized in bacteria?
- most structural genes w related functions are organized into operons for efficiency of gene expression
- this means that transcription of the set of genes in the operon is controlled by a single promoter and a regulatory sequence called “operator”
- operators can bind regulatory proteins
What is a negative inducible operon?
= if NO inducer present: regulatory protein is a repressor biding operator and prevents transcription of the structural genes in the operon
= if inducer present: binds to regulator making it unable to to bind to the operator and transcription takes place
(lac operon works like this)
How does a negative repressible operon work?
= No product present: regulator protein is inactive repressor and unable to bind to operator, transcription of structural genes therefore takes palce
= Product present: the product binds to the regulatory protein and makes it active and able to bind to the operator, this prevents transcription
What is positive control of bacterial gene expression?
= regulatory protein is an activator, binds to DNA and interacts with RNA polymerase to assist in efficiency of transcription of structural genes
- lac operon is a negatively inducible but positively-regulated operon
What is co-ordinate expression?
= the fact that structural genes can be controlled as a single unit when they are grouped together in an operon
When are the genes of the lac operon transcribed?
= when lactose is present and glucose is absent
Where is the lac operon located?
b/n purE and proA genes on bacterial chromosome
What does the lac operon consist of?
- lac repressor lacI gene w its own promoter - PI) = regulator gene
- beta-galactosidase gene (lacZ)
- beta galactoside transacetylase gene (lacA)
- transcription of lacZ, Y and A genes controlled by single lac promoter (P) and lac operator (O) sequence
- discovered by F Jacob and J Monod in 1961 (Nobel prize 1965)
What does the Y gene encode for in lac operon?
= lactose permease, a membrane protein that transports lactose into bacterial cell
What does the Z gene encode for?
= beta galactosidase, main purpose to cleave lactose into glucose and galactose
- low background level converts lactose to allolactose (the lac operon inducer)
What does the lac I region encode for?
= lac repressor: protein that inactivates the lac operon in absence of the inducer allolactose
- repressor is a diffusable tetramer
What does the lac O region encode for?
= the lac operator: a regulatory DNA sequence that works in cis (=lacO only affects regions downstream) to control lac operon, lac repressor can bind to the lac operator
Where is the lac O region located?
= b/n lac promoter and lacZ gene
How does the lac repressor protein work?
= binds to the lac operator (lacO) when there is no lactose (no allolactose inducer), this blocks the RNA polymerase from binding to the -35(binding) and -10(unwinding) sequences (promoter region) in the lac promoter b/c lac operator overlaps the initiation start
- means structural genes cannot be transcribed (lac Z, Y, and A genes) = OFF characteristic of lac operon
How is the lac operon turned ON?
- lac repressor binds inducer and is inactive
- lac operon is induced and genes Z, Y, and A are transcribed and translated
What convention is used to denote the normal (wildtype) and mutant forms of the lac operon genes?
+ for wildtype and - for the mutant gene
so of I+P+O+Z+Y+A+ then allolactose is inducible for all lac operon enzymes
but if I+P+O+Z+Y-A- then allolactose is inducible only for lacZ
