Genes are able to replicate horizontally and vertically- a given gene
may be more successful on horizontally mobile elements than others. This
includes genes that exhibit a post-segregational killing (PSK) phenotype.
PSK is generated by expression of a toxin and antitoxin from a mobile
element, such that if a bacterium loses the element the toxin becomes
active in the cell and the cell dies. All PSKs described to date involve a
toxin and an antitoxin function, though within a given group of toxin and
antitoxin gene pairs only some are likely to exhibit this phenotype. Here, I
investigate what differentiates genes that induce PSK from biochemically
similar genes that do not.
One group of genes of which some are known to induce PSK is toxinantitoxin
(TA) systems, composed of a stable toxin and an unstable
antitoxin. I analyzed computational data on the distribution of type I TA
systems (RNA antitoxin), which appear to be less mobile than type II
TA systems (protein toxin). Data on validated TAs suggests a correlation
between distribution, mobility and the PSK phenotype. Differences in
phylogeny could be due to differences in tendency to exhibit PSK in different
environments. This connection between distribution and PSK was
explored by experimentally testing a computationally described operon,
plasmid_Toxin-ptaRNA1, that exhibited structural and distributional
similarities to a mobile type I TA system. Despite this, expression of the
predicted toxin ORFs did not reduce growth (as measured by saturation
density) in E. coli, and the operon did not induce PSK.
The conditions of PSK were further tested with the toxin (barnase)
and antitoxin (barstar), which are not known to have the phenotype. A
number of heterologous expression systems were developed with these
genes in E. coli to test their ability to exhibit PSK in a manner akin
to both type II TA systems, with a cytoplasmic toxin, and bacteriocins,which have a secreted toxin. I used equations of logarithmic decay to
model the necessary expression of the proteins in the cell and their rate
of decay after plasmid loss to enable PSK. My results suggest there is
likely to be an evolutionary trend toward TA systems with high expression
levels of very unstable antitoxins. Secreted barnase was also tested
experimentally for its ability to induce PSK similar to bacteriocins, which
exhibit a PSK-like phenotype in monoculture by driving maintenance of
the immunity encoding plasmid. Barnase did not induce PSK, possibly
due to its inability to cause antibiosis in our test system.
Structural similarities and biochemical similarities are not sufficient to
determine whether a given system will act as a PSK because numerous
contextual factors have an effect on whether the genes are addictive.
A given set of genes may have the phenotype in one species but not
another, under one set of environmental conditions but not another, or
on one replicon but not another. This is consistent with the competition
hypothesis, which states that genes will be selected for on mobile elements
due to their ability to increase horizontal reproductive success, depending
on the environmental conditions.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/9932 |
| Date | January 2014 |
| Creators | Coray, Dorien Skye |
| Publisher | University of Canterbury. Biological Sciences |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Dorien Skye Coray, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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