Creation of a viable csrA mutant in Vibrio cholerae

Abstract

Vibrio cholerae, the causative agent of cholera, has been a lethal enteric pathogen to humans for most of recorded history. Even though it is well studied, it still kills many people every year due to rapid and severe dehydrations from diarrhea. Part of what makes V. cholerae such an effective pathogen is its ability to control virulence factors depending on its environment. ToxR is a major virulence protein that has upstream control of most of the virulence genes that are turned on when in a human host. Two of the most critical virulence factors, toxin coregulated pilus and cholera toxin are controlled by ToxR. CsrA is a protein that regulates many cellular functions in V. cholerae, including glycogen synthesis, motility, and biofilm production. Preliminary data suggests a link between CsrA and the regulation of ToxR. In order to study CsrA as it relates to ToxR regulation, a csrA mutant must be generated in V. cholerae. CsrA plays such an important role in glycogen metabolism that a csrA mutant is not viable due to excessive glycogen levels. In order to make a viable csrA mutant, glycogen synthesis has to be turned off. In this research, I attempt to make a viable V. cholerae csrA mutant by deleting csrA in a strain that is deficient for glycogen synthesis (glg). Normally without CsrA, glycogen in the cell would increase to a detrimental level. Since a glg⁻ csrA⁻ mutant lacks the ability to make glycogen, the levels never reach a lethal level, allowing the mutant to survive without functional CsrA. Such a glg- csrA- double mutant's ToxR regulation can be studied by growth in various media by measuring OmpU and OmpT expression. Using PCR, restriction enzymes, and DNA ligase, a suicide plasmid was created containing sequences that flank the csrA gene but instead of the csrA gene, a chloramphenicol resistance cassette was inserted. Through bacterial conjugation this plasmid was introduced into three V. cholerae glg- strains. Allelic exchange was carried out utilizing the homology between the DNA flanking wild type csrA and the csrA deletion with chloramphenicol cassette. This first crossover event was initiated with the requirement of the [pi] protein for the plasmid to replicate. Without the pir gene to create [pi] protein, selection for antibiotic resistance required that the plasmid integrate into the genome. This was selected based on the plasmid encoded ampicillin resistance. After the second crossover event, there were two possible outcomes of excision: reverting to wild type csrA or retention of the csrA mutation. The csrA mutant was selected based on its sucrose and chloramphenicol resistance and ampicillin sensitivity.