Our research is directed toward elucidating the evolution of
bacterial genomes, including their size, composition, variability
and organization. In other words, why do genomes have the
genes that they do? An understanding of the evolutionary
process that leads to differences in genomes will shed light on
how species themselves differentiate. We take two approaches
to understanding how genomes evolve : a
computation/theoretical approach, and an
The enteric bacteria provide a set of well characterized
species with notable structural and biochemical differences. The
variable genetic components of these species provide a record of
the current and historical selective influences leading to
speciation. Several ongoing projects are addressing substantial
metabolic differences among closely enteric bacteria, each
focusing on a different process affecting genome
- Cobalamin synthesis in Klebsiella.
Escherichia coli, Salmonella enterica and Klebsiella
aerogenes all synthesize cobalamin (coenzyme B12) in substantially different ways. The
systems in E. coli and Salmonella have been
studied previously. We are using genetic and molecular
approaches to understand how and why Klebsiella
synthesizes cobalamin, and why this pathway was lost in the
ancestor of E. coli and Salmonella. Horizontal
gene transfer - at least two separate events - plays a significant
role in the evolution of this gene cluster among these taxa.
- Methionine recycling in
Klebsiella. Unlike E. coli and
Salmonella, we have found that Klebsiella uses two
distinct pathways to recycle the sulfur atom from methionine for
use in cysteine (the key sulfur donor for biosynthesis). We are
using genetic, molecular and biochemical approaches to
understand how and why Klebsiella performs these
metabolic feats, which may tell us why the ancestor of E.
coli and Salmonella lost these genes as well.
- Antigenic diversity in Salmonella. Salmonella is
very antigenically diverse (strains are very different on the
outside). Although conventional wisdom says the diversity allows
this pathogen to avoid the immune system, this model doesn't
explain how Salmonella became diverse in the first place.
We are examining the role of protozoan predation in the origin
and maintenance of antigenic diversity in Salmonella,
proposing that diversity allows this species to avoid predation by
amoebas as well as white blood cells.
We also use bioinformatic approaches to glean evolutionary
histories from bacterial genome data. Some of the questions
currently being examined include the following:
- What controls the rate of horizontal gene transfer?
Horizontal transfer has had a tremendous impact in the evolution
of enteric bacterial genomes. Is this effect widespread? Why is
the rate higher in some genomes and lower in others? We have
developed biostatistical measures that can predict the rate of
horizontal transfer based on genome sequence signatures.
- How does horizontal transfer affect bacterial
speciation? The introduction of novel functions by horizontal
transfer, like the cob and pdu operons into
Salmonella, allow rapid expansion into new niches.
Saltation in phenotypic states may provide the environmental
separation necessary for bacterial speciation. How many of the
differences distinguishing closely related taxa may be attributed to
- What are the constraints on bacterial genome size?
Bacterial genomes have long been considered streamlined, that
is reduced in size for rapid growth. Yet natural variation in
genome size within a species and the propensity for large
duplications in growing strains both belie this conclusion. Rather,
it is likely that bacterial genomes may be as large as is possible for
a species to maintain, depending on its effective population size.
Is there a correlation between effective population size and
genome size among bacterial taxa?
Last Updated 14 August 2006, by JG Lawrence