Biology Reference
In-Depth Information
GENOMICS OF THE OBLIGATE SYMBIONT
WIGGLESWORTHIA
Each tsetse Þy harbors about 1
10
cells of
Wigglesworthia
in its bacteriome, with bacteria lying
8
free in the cytoplasm of the bacteriocytes
(
from several distant tsetse host species had been estimated
to range from 705 to 770 kilobases (kb) based on pulsed-Ýeld gel electrophoresis analysis (Akman
and Aksoy, 2001). Recently, its completely sequenced genome was found to be 697,724 base pairs
(bp) in
The genome size of
Wigglesworthia
Wigglesworthia brevipalpis
(Akman et al., 2002)
Ð
about one sixth of that of the related
free-living
(4.6 megabases [Mb]). Similar drastic genome size reductions have
been observed for intracellular pathogens such as
Escherichia coli
Chlamydia trachomatis
(1.04 Mb),
Treponema
pallidum
(1.14 Mb),
Mycoplasma genitalium
(0.58 Mb) (Fraser et al., 1995), and
Rickettsia
prowazekii
(640 kb) (Charles
and Ishikawa, 1999; Shigenobu et al., 2000). The genome sizes of the mutualists
(1.1 Mb), as well as for the primary endosymbiont of aphids,
Buchnera
Wigglesworthia
and
smallest bacterial genome
reported thus far. As a reÞection of gene loss (and presumably loss of their associated functions),
neither of the modern-day obligates can live outside its host-insect niche.
Another hallmark of intracellular bacteria is the high A+T content in their coding sequences.
The
Buchnera
are apparently approaching that of
M. genitalium,
the
Wigglesworthia
genome has an average A+T content of 78% and the
Buchnera
genome is 74%
(Shigenobu et al., 2000), while the overall A+T content of the genomes of
R. prowazekii
,
M.
genitalium,
are 71, 68, and 59%, respectively. In contrast, the A+T content of
free-living bacteria are lower, such as 45% in
and
C. trachomatis
(Blattner et al., 1997). Analysis of the coding
capacity of these reduced genomes has indicated that loci encoding for DNA repair and recombi-
nation functions have been lost or limited in many cases (Moran and Wernegreen, 2000). It is
thought that this loss of the repair functions might have led to the A+T bias observed in these
intracellular genomes (Moran, 1996).
The completely annotated sequence of
E. coli
genome has revealed the presence of
621 predicted coding sequences (CDSs) with an average length of 988 bp. It has been possible to
assign biological roles to 522 (86%) of these putative proteins, while 95 proteins (14%) matched
hypothetical proteins of unknown function. Comparative analysis of the CDSs indicates that the
Wigglesworthia
Wigglesworthia
genome contains a subset of the genes of free-living bacteria, such as the enteric
E. coli
, further supporting the idea that it shares an ancestor with them.
One of the surprising Ýndings of the
and
Salmonella typhimurium
genome was the absence of the important
gene coding for the DNA replication-initiation protein, DnaA Ð an observation unprecedented in
eubacteria. As would be expected, its genome also did not exhibit any DnaA boxes or a clear GC
Wigglesworthia
A
B
Bac
FIGURE 4.2
(A) The
U
-shaped bacteriome (Bac) structure in the anterior midgut. (B)
Wigglesworthia
cells
lying within the bacteriocyte cytoplasm. Bar = 5
n
m.
