Year of Award

2008

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Other Degree Name/Area of Focus

Microbiology, Biochemistry

Department or School/College

Division of Biological Sciences

Committee Chair

Michael F. Minnick

Commitee Members

Keith Parker, Michele McGuirl, Scott D. Samuels, Stephen J. Lodmell

Keywords

coxiella burnetii, horizontal gene transfer, intein, intervening sequence, intron

Abstract

The genome of the obligate intracellular pathogen Coxiella burnetii contains a large number of mobile genetic elements including two group I introns and an intervening sequence (IVS) that interrupt the 23S rRNA gene; an intein within dnaB (encoding replicative DNA helicase) and a homing endonuclease. The introns are self-splicing ribozymes and able to inhibit ribosome function and retard bacterial growth through internal guide sequence (IGS)-dependent and -independent mechanisms. The introns were found to be highly conserved in all eight genomic groups of C. burnetii, suggesting a role in C. burnetii's biology. It is not clear whether the introns are being positively selected because they promote bacterial persistence or whether they are slightly deleterious but were fixed in the population due to genetic drift.

The intein is able to self-splice, leaving the host protein intact and presumably functional. Similar inteins were found in two extremophilic bacteria (Alkalilimnicola ehrlichei and Halorhodospira halophila) that were found to be distantly related to Coxiella. Intein splicing appears to be a slow process, making it possible that before the intein is excised the DnaB precursor is non-functional, thereby reducing the pool of mature DnaB, thus creating a lag in replication and slowing growth.

The IVS is found inserted into helix 45 of C. burnetii's 23S rRNA. Unlike introns and inteins, the flanking regions are not spliced back together after the IVS is excised, resulting in a fragmented 23S rRNA. The IVS encodes a hypothetical protein that is conserved between IVSs in a wide variety of bacteria. Phylogenetic analyses revealed that a similar ORF is present on an IVS inserted at the same locus in many Leptospira species, suggesting horizontal gene transfer as the mode of spread of this genetic element. It is possible that the dramatic drop in ribosome quantities that occurs when Coxiella transitions from LCV to SCV is aided by RNA fragmentation caused by the IVS. Another mobile genetic element found in C. burnetii is a homing endonuclease encoded within an intron. Here, we have characterized four "selfish" genetic elements in C. burnetii and show that they share an intimate relationship with their host.

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© Copyright 2008 rahul raghavan