Description:

Microbiology-Immunology Seminar Series

"The obligate human pathogen, Neisseria gonorrhoeae, expresses the pilin antigenic variation system (pilin Av), which alters the sequence of the major pilin, PilE, allowing for immune evasion. Formation of an alternate DNA structure called a guanine quadruplex (G4) upstream of pilE, is required for pilin Av. This 16 base guanine-rich motif contains four tracts of guanines, with one or two loop thiamine residues. Each of the 12 guanines is required for Av, however the other requirements of this structure for Av are unknown. It was previously reported that the RecA recombinase required for Av, binds the pilE G4, and that alternative G4 forming sequences cannot substitute for the pilE G4 for pilin Av. I have shown that RecA binds these alternative G4 structures with similar affinity, demonstrating that RecA binding is not the reason other G4 structures cannot replace the native structure. I then investigated the folding kinetics and stability of these alternate G4 structures and found the kinetics do highly correlate with the pilin Av phenotypes. The pilE G4 is the most stable structure and allows for the highest rates of pilin Av. Additionally, transcription of a cis-acting, non-coding, sRNA that initiates within the G4 forming sequence is necessary for pilin Av. I have shown that lowered transcription levels result in lower Av frequency, suggesting that sRNA transcription is a rate-limiting step for pilin Av. However, insertion of a transcriptional stop in several places downstream of the G4 sequence does not alter the level of pilin Av, suggesting that an extended transcript is not required for pilin Av and supporting the hypothesis that the activity of the sRNA is localized to the G4 sequence. These studies will further elucidate a role for the G4 structure and sRNA in the complex mechanism of pilin Av."

Lauren Priniski

Northwestern University Driskill Graduate Program

Advisor: Dr. Hank Seifert