Unit 3--Lecture 15 (Microbial Regulatory Systems P1) Flashcards
Regulating Gene Expression
Microbes respond to changing environment
—-alter growth rate, proteins produced, and behavior
Must sense their environment
—-need receptors to transmit information to chromosome
Need to change enzyme function
—-transcriptional, translational, and post-translational mechanisms
Sensing the Environment
Sensor kinase protein in plasma membrane
—-binds to signal
Cytoplasmic response regulator
—-alters transcriptional rate of chromosomal genes
Altering Transcription Rates
Sigma factor initiates transcription by RNA at the promoter
Regulatory proteins bind to operator sequences
Activators bind to the operator and increase the strength of that gene’s transcription
Repressors bind to the operator and lower the strength of that gene’s transcription
E. coli Lac Operon
Lactose (milk sugar) is used for food
—-cannot pass through plasma membrane
—-lactose permeate allows entry
—–PMF used to bring lactose inside cell
—-must be converted to glucose to be catabolized
Operon: A group of coordinately expressed and regulated genes associated with a common purpose
lacZ gene encodes B-galactosidase
lacY gene encodes lactose permease
lacA gene encodes a transacetylase
E. coli Lac Operon (- lactose)
Repressor protein LacI blocks transcription
—-repressor binds to operator
—-blocks sigma factor from binding promoter
E. coli Lac Operon (+ lactose)
Repressor responds to presence of lactose
—-binds inducer (allolactose) or DNA, not both
—-add lactose –> repressor falls off operator
Catabolite Repression
Operon enabling the catabolism of one nutrient is repressed by the presence of a more favorable nutrient (commonly glucose)
The biphasic curve of a culture growing on two carbon sources is called diauxic growth
Glucose is the easiest sugar to catabolize
Glucose is transported using a phosphotransferase system
Presence of glucose affects an internal signal (cAMP)
IIA^(Glc) inhibits adenylate cyclase
IIA^(Glc) reduces internal cAMP pool
High glucose –> low cAMP
cAMP Affects Transcription
Maximum expression of the lac operon requires the presence of cAMP and cAMP receptor protein (CRP)
CRP is an inducer of the lac operon
Inducer Exclusion
Glucose transport also inhibits lactose transport
IIA^(Glc) uncouples LacY
Arabinose Operon Control
ara operon
AraC acts as repressor to block transcription
Addition of arabinose changes conformation
—-now acts as activator
—-stimulates binding of RNA polymerase
Trp Operon Transcription
trp operon contains 5 genes to make tryptophan
Only express in the absence of tryptophan
Opposite of lac repressor
Trp aporepressor must bind tryptophan in order to bind the operator as the holorepressor
Attenuation of the Trp Operon
Attenuation: a regulatory mechanism in which translation of a leader peptide affects transcription of a downstream structural gene
The attenuator region of the top operon has 2 trp codons and is capable of forming stem-loop structures
Mechanism of Attenuation
Riboswitches
Metabolite directly bound by the mRNA
Induced conformational change
Results in either:
—-transcription termination
—-ribosome exclusion
—-mRNA degradation
Control of Bacteriophage Lambda
Lytic Cycle
-Phage quickly replicates, kills host cell
-Generally lytic when host cell conditions are good or are very bad (cell damaged)
Lysogenic Cycle
-Phage is quiescent
-May integrate into host cell genome
-Replicates only when host genome divides
-Generally lysogenic in moderate cell conditions
-Phage can reactivate to become lytic, kill host
