Biology Reference
In-Depth Information
skewing
.
The lack of an autonomous DNA-replication machinery may reÞect the dependency of
Wigglesworthia
on its tsetse host genome functions and may be one of the mechanisms by which
the host bacteriocyte cell regulates symbiont numbers.
does not infect the egg tissue but is maternally transmitted to tsetse intrauterine
larva via milk-gland secretions. It is not clear, however, whether whole bacteriocytes or
Wigglesworthia
Wiggles-
worthia
cells are transferred from the mother to her larva. While the commensal symbiont
Sodalis
has been shown to utilize a Type III secretion system to invade the larval tsetse cells (Dale et al.,
2001), the
Wigglesworthia
genome does not encode for a Type III secretion system that could be
used for invasion. The
genome, however, has retained the machinery for the
synthesis of a complete Þagellar apparatus including the hook, Ýlament, Ýlament cap regions, and
the integral membrane proteins required for motility functions,
Wigglesworthia
. While retention of
the genes associated with the Þagellar operons is suggestive of its functional role, neither Þagellum
nor motility has been observed in
motA
and
motB
in adult bacteriocytes. It remains to be seen
whether the Þagellum is expressed in the different development stages of tsetse. It is possible that
components of the
Wigglesworthia
Þagellum may provide an alternate to the Type III secretion
mechanism of cell invasion to enable entry into the host larval or pupal gut cells destined to be
bacteriocytes. A mechanism utilizing the Þagellar export system has been shown for
Wigglesworthia
Ye rsinia
invasion of mammalian gut cells (Young et al., 1999). Alternatively, since the
Wigglesworthia
genome encodes for very few transporters and has only a partial sec-dependent export pathway,
components of the Þagellar structure may fulÝll export functions, as has been suggested in the case
of
(Shigenobu et al., 2000).
Among the physiological traits shared by obligate genomes that set them apart from small
pathogens is their ability to supplement their hostÔs genome needs. For example,
Buchnera
provide
all essential amino acids that the host diet apparently lacks. Similarly, the small genome of
Buchnera
Wigglesworthia
has retained the ability to synthesize various vitamin metabolites including biotin,
thiazole, lipoic acid, FAD (riboÞavin, B2), folate, pantothenate, thiamine (B1), pyridoxine (B6),
protoheme, and nicotinamide, which are known to be low in the single diet of tsetseÏvertebrate
blood. Hence, supplementing the eukaryotic diet with vitamin products
may play a central role in
the functional basis of this mutualistic relationship.
Despite the apparent functional and evolutionary similarities of their symbiotic associations,
the genetic blueprints of the obligates
Buchnera
and
Wigglesworthia
are quite distinct.
Wiggles-
worthia
, and these represent mostly the indispensable
housekeeping genes involved in transcription, translation, and cellular functions
(Figure 4.3).
Also, in comparison to
shares only 69% of its CDSs with
Buchnera
genome
encodes for lipopolysaccharides and phospholipids, components necessary for a Gram-negative
cell-wall structure, and for a full Þagellar structure. Since both of these characteristics are
associated with free-living or parasitic microbes such as
Buchnera
, a signiÝcant portion of the small
Wigglesworthia
Rickettsia
, either
Wigglesworthia
rep-
resents a much Ñyounger symbioticÒ association than a true obligate like
or its unique
route of intrauterine transmission may have shaped its genome to retain functions associated
with parasitic or free-living microbes.
Buchnera
GENOMICS OF GENUS
SODALIS
, the secondary symbiont of tsetse, shows similarities to the free-living enterics. Its genome
size is approximately 2 Mb, which is signiÝcantly larger than those of the intracellular pathogens
and obligate symbionts yet smaller than those of the closely related free-living enterics (Akman
et al., 2001). In addition,
Sodalis
cells harbor an abundant large plasmid of about 135 kb. Based
on the
groEL
and
ftsZ
gene sequences, the A+T content of the
Sodalis
genome is less than 45%,
which is more similar to free-living organisms, unlike the A+T-rich intracellulars described earlier.
Although genome-wide sequence data are not yet available for
Sodalis
, hybridization of its
genomic DNA to
E. coli
macroarrays revealed the presence of 1800 orthologs (ORFs), which
Sodalis
