Bacterial Genetics Flashcards
T or F. Bacteria encode a single RNA polymerase responsible for transcribing all needed RNAs
T
What is the region upstream of every coding sequence and is that spot for RNA polymerase binding?
the promoter region
What is an operator?
generally located near the promoter, and is a sequence that binds proteins involved in the regulation of the gene’s expression
One significant difference between eukaryotic and prokaryotic genes is the ________.
lack of introns in prokaryotic coding sequences.
This feature, plus the lack of a nucleus in bacteria, allows for the coupling of transcription and translation in bacteria: translation of mRNAs begins before transcription of the message is completed.
What is an operon?
a cluster of genes, typically involved in a related function, that are transcribed on a single mRNA (i.e., a polycistronic message where cistron=gene), and are then translated independently to produce the individual proteins.
What is the lac operon?
a cluster of genes involved in the utilization of lactose by the Gram negative bacillus Escherichia coli.
Thus, in the case of the lac operon, there is a single promoter and operator upstream of the first gene, lacZ, and a single transcription termination site downstream of the last gene, lacA. All three units (lacZ, lacY, and lacA) contain their own ribosomal binding sites to independently translate their portion of the mRNA to produce the corresponding proteins
What are the corresponding proteins of lacZ, Y, and A on the alf operon of E. coli?
B-galactosidase,
lactose permease, and
lactose transacetylase, respectively.
How is the translation of proteins in bacteria different from that in viruses?
The translation of individual proteins from a polycistronic RNA template in bacteria can be contrasted with a process seen in certain RNA viruses of animals where a polycistronic RNA template is translated into a single “polyprotein” that is then cleaved into individual proteins
The clustering of functionally-related genes into operons serves at least two purposes. What are they?
(i) operons help to ward off loss of function because if genes contributing to a single process reside in different regions of the genome, they can be more easily lost to evolutionary divergence, and
(ii) the use of a single promoter and operator provides co-regulation of the genes in the operon, facilitating coordinate expression of functionally-related genes.
What are the three types of gene regulation in bacteria?
(i) constitutive, where there is effectively no regulation, the genes are always expressed,
(ii) positive regulation, in which an activator protein promotes RNA polymerase binding to promoters and therefore facilitates expression, and
(iii) negative regulation, in which a repressor protein binds the operator sequence and prevents transcription by RNA polymerase until the repressor is removed.
NOTE: Because transcription and translation expend energy, constitutive expression is rare; most wild type genes are regulated.
What do inducers do?
Regulation is often provided through the use of inducers, molecules that either interact with activator proteins, causing them to bind at operators and promote RNA polymerase binding at nearby promoters, or interact with repressor proteins, preventing them from binding operators and subsequently blocking transcription by RNA polymerase.
T or F. Negative regulation appears to occur less often than positive regulation.
F. Negative regulation appears to occur more often than positive regulation. If you consider that most mutations are deleterious, this makes sense: loss of a repressor allows continued gene expression, but loss of an activator leads to loss of gene expression, and hence, function.
What does the lacI gene do?
the lac operon and encodes a repressor protein; lacI is constitutively expressed.
n the absence of lactose in the growth medium, the repressor protein is found predominantly bound to the lac operator, where it prevents RNA polymerase from transcribing the operon.
This makes sense – in the absence of lactose, it would be wasteful, in energy terms, to transcribe and translate the lac proteins when the sugar substrate is unavailable. The energy expenditure would not be recouped.
What happens if lactose is present?
However, if lactose is present in the growth medium, catabolism of the lactose would lead to a net gain in energy for the bacteria. Therefore, in the presence of lactose, the sugar is transported inside the cell where it is converted to a related structure, allolactose, which serves as the inducer of lac expression
How does allolactose induce lac expression?
As with any bound ligand, the lacl repressor releases from the operator at low frequency, and when this occurs in the presence of the inducer allolactose, the allolactose binds the repressor protein, altering its conformation, an outcome termed allostery. The altered conformation prevents the repressor from binding the lac operator, thus freeing the operon from repression.
What happens once the lactose has been used up?
Once the lactose has been used up from the growth medium, allolactose will no longer be formed, lac repressor molecules will no longer be allosterically altered, and they will return to the operator site to again repress lac operon expression.
While operons are an efficient structure for co- regulation, functionally-related genes do not always cluster into a single operon. When independently transcribed genes, or multiple operons, are controlled by the same regulatory protein or process, they are considered to constitute a _____.
regulon
What is the two-component signaling pathway of bacteria?
named because of a sensor found in the cytoplasmic membrane and an intracellular transducer
The binding of ligand to the sensor leads to the sensor’s phosphorylation and subsequently to the phosphorylation of the intracellular transducer. Once phosphorylated, the transducer is then able to function as an activator and bind the operator of genes whose expression will lead to an appropriate response to the extracellular ligand.
What is the advantage of the two-component signaling pathway?
This represents a way that bacteria can respond to changes in extracellular conditions without the need to transport the ligand inside the cell, as is the case in lac operon regulation. Changing temperature, osmolarity, or availability of extracellular iron are examples of conditions in which two-component regulatory systems are used by bacteria to respond appropriately.
How do bacteria communicate with each other?
by releasing specific signaling molecules and by measuring the local concentrations of signaling molecules in their environment. As the cell density of the population increases, the concentration of signaling molecules also increases.
