Biology Reference
In-Depth Information
leucine and tryptophan biosynthetic genes have ampliÝed on plasmids, leading to an overproduction
of these amino acids (Bracho et al., 1995; Baumann et al., 1997; Baumann et al., 1999). The
importance of tryptophan overproduction by
has been experimentally demonstrated
(Douglas and Prosser, 1992). In this study, pea aphids were cured of the bacteria by chlortetracycline
treatment and fed a diet deÝcient in tryptophan. Under these circumstances the pea aphids rarely
reached adulthood.
The key enzyme in the metabolic pathway leading to tryptophan biosynthesis is anthranilate
synthase. The two subunits of the enzyme are encoded by the
Buchnera
trpEG
genes that are ampliÝed in
the form of tandem repeats amounting to 3.6 kb of the
genome (Sg) (Lai et al., 1994).
The tandem repeats account per se for a 16-fold ampliÝcation, not including additional effects
caused by high plasmid copy numbers. The remaining genes dedicated to tryptophan biosynthesis
(
Buchnera
) are located on the main chromosome (Munson and Baumann, 1993). The same
pattern is seen in several
trpDC(F)BA
species from the Aphididae, indicating that the ampliÝcation
occurred prior to the divergence of this group of aphid endosymbionts (Lai et al., 1996). However,
this pattern is not universal among the aphid endosymbionts because the
Buchnera
genes are located
on the chromosome in at least one representative of the Pemphigidae (van Ham et al., 1999). The
most likely explanation is that a single ancestral transfer of
trpEG
trpEG
to a
RepA
/
C
-like replicon took
place, followed by a reverse transfer to the chromosome in some
Buchnera
lineages (Roeland et
al., 1999; van Ham et al., 2000).
Genes involved in the biosynthesis of leucine are also located on plasmids (
leu
ABCD) in
Buchnera
(Bau-
mann et al., 1999). These plasmids vary in size, from 3.0 to 12.8 kb, and contain two additional
genes,
residing in
Schizaphis graminum
,
Diuraphis noxia
, and
Rhopalosiphum padi
repA
1 and
repA
2, involved in plasmid replication (Lai et al., 1994). There are 24 copies
of the leucine genes in
Buchnera
(Sg) when compared to the single copy identiÝed in
Buchnera
of
.
The presence or absence of plasmid ampliÝcation in particular lineages is perhaps connected
to different needs for tryptophan/leucine, which is related to differences in the overall life cycle
such as developmental time and reproduction rates (Lai et al., 1995). The origin of the tryptophan
and leucine biosynthetic plasmids in
D. noxia
Buchnera
is not clear, as they are not universally present in
all
strains of the Aphidae families. In addition, different gene contents and orders have
been reported in different strains of
Buchnera
(Soler et al., 2000). The process of relocation and
ampliÝcation of these amino-acid biosynthetic genes on plasmids is most likely relatively novel in
evolutionary terms and at least not older than the establishment of the endosymbiotic relationship.
Buchnera
BUCHNERA
AND THEIR FREE-LIVING RELATIVES
Members of the Enterobacteriaceae, such as
E. coli
and
Haemophilus inÞuenzae,
are the closest
modern free-living relatives of
, members of Enterobacteriaceae are
commensals, which is reÞected in genome sizes, contents, and architectures. For example, there is
a considerable genome size variation among
Buchnera
. Unlike
Buchnera
E. coli
strains. Currently, there are three sequenced
E. coli
genomes: the K-12 strain with a genome size of 4.60 Mb, the 0157:H7EDL strain with a
size of 5.50 Mb, and the 0157:H7 strain totaling 5.60 Mb (
www.ncbi.nlm.nih.gov/entrez/query.fcgi).
These differences appear to be largely explained by the presence or absence of bacteriophages,
prophages, and virulence genes in the genome (Ohnishi et al., 2001). In between the variable parts
is a well-conserved core of the
E. coli
chromosome with no large rearrangements. This ÑbackboneÒ
of the
genome, which is estimated to be around 4.1 Mb, is topologically fragmented by
strain-speciÝc pieces distributed around the chromosome (Hayashi et al., 2001).
The
E. coli
0157 genome (Shigenobu et al.,
2000; Tamas et al., 2002). There are no examples of horizontally transferred DNA or any obvious
virulence-associated genes, suggesting that the
Buchnera
genomes are less than one eighth of the
E. coli
Buchnera
genome is essentially a small subset of the
E. coli
genome. The loss of genes is evident in all categories but particularly so in categories such as
