Streptococcus parasanguis is a bacterial species that...

Streptococcus parasanguis is a bacterial species that initiates dental plaque formation by adhering to teeth. To investigate ways to climinate plaque, rescarchers constructed a plasmid, depicted in the figure shown, to mutagenize $S$. parasanguis. The key features of this plasmid include rep$A^{\text {ss}}$ a temperature-sensitive origin of replication $), k a n^{r}$ (a gene for resistance to the antibiotic kanamycin), and the transposon IS256. This transposon contains the $e r m^{r}$ gene for resistance to the antibiotic erythromycin. IS256 transposes in $S$. parasanguis a. How could the researchers use this plasmid as a mutagen? Consider how they could get the transposon's into the bacteria, and how they could identify strains that had new insertions of $I S 256$ into $S$. parasanguis genes. Your answer should explain why the plasmid has two different antibiotic resistance genes as well as a temperature-sensitive origin of replication. b. Why would the researchers use this plasmid as a mutagen? c. If the investigators found a mutant strain of $S$. parasanguis that was defective in plaque formation, how could they identify the affected gene?

Streptococcus parasanguis is a bacterial species that initiates dental plaque formation by adhering to teeth. To investigate ways to climinate plaque, rescarchers constructed a plasmid, depicted in the figure shown, to mutagenize $S$. parasanguis. The key features of this plasmid include rep$A^{\text {ss}}$ a temperature-sensitive origin of replication $), k a n^{r}$ (a gene for resistance to the antibiotic kanamycin), and the transposon IS256. This transposon contains the $e r m^{r}$ gene for resistance to the antibiotic erythromycin. IS256 transposes in $S$. parasanguis
a. How could the researchers use this plasmid as a mutagen? Consider how they could get the transposon's into the bacteria, and how they could identify strains that had new insertions of $I S 256$ into $S$. parasanguis genes. Your answer should explain why the plasmid has two different antibiotic resistance genes as well as a temperature-sensitive origin of replication. b. Why would the researchers use this plasmid as a mutagen?
c. If the investigators found a mutant strain of $S$. parasanguis that was defective in plaque formation,
how could they identify the affected gene?

Key Concepts

Transposon Mutagenesis

Transposon mutagenesis is a genetic technique used to create random mutations by inserting a mobile genetic element, or transposon, into a genome. The inserted transposon disrupts genes at its insertion site, which can lead to loss-of-function mutations. This method is valuable for identifying gene function and linking genetic elements to specific phenotypes in an organism.

Temperature-Sensitive Plasmids

Temperature-sensitive plasmids are engineered to replicate only under specific temperature conditions. They contain an origin of replication that is active at a permissive temperature and inactive at a non-permissive (higher) temperature. This feature allows researchers to control the persistence of the plasmid within the host, thereby facilitating the selection for genomic insertion events, such as the movement of a transposon, once the plasmid is lost at the restrictive temperature.

Antibiotic Resistance Markers

Antibiotic resistance markers are genes that confer resistance to specific antibiotics, allowing for the selection of cells that have taken up genetic material. In the context of mutagenesis, different markers can distinguish between cells that have acquired the plasmid and those in which the transposon has inserted into the genome. This dual marker system allows researchers to isolate and identify cells with successful transposition events based on growth in the presence of the corresponding antibiotics.

Mutant Screening and Gene Identification

Mutant screening involves examining a library of genetically modified organisms to identify those with a desired phenotype, such as a defect in a specific biological process. Once a mutant exhibiting the phenotype is isolated, molecular techniques such as mapping, PCR, or sequencing can be employed to identify the disrupted gene. This approach is fundamental for linking gene function to phenotypic outcomes and understanding the genetic basis of biological processes.

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