Biology Reference
In-Depth Information
TABLE 3.2
Examples of Buchnera Pseudogenes in Three Functional
Categories
Function
Buchnera (Sg)
Buchnera (Ap)
Amino acid metabolism
Cysteine biosynthesis
cysNDGHIQ
Asparaginase
asnA
Methionine regulation
metR
Restriction-repair systems
Base excision repair
endA, lig, ung
Uvr excision repair
mfd
Direct damage reversal
phrB
Membrane components
Lipoproteins
lgt , nlpD, mrcB
Phospholipids
fabZ
fabD
Peptidoglycan
murCEF, mraY, mltE
ddlB
Description of pseudogenes: In addition to pseudogenes that show clear homol-
ogy to functional genes in other species, some sequences show signs of more
extensive degradation and are recognizable as gene remnants only through short
stretches of homology plus a conserved position and orientation relative to
Þanking, intact genes. Among these are Ýve genes ( ansA, hemD, apbE, cmk ,
and cvpA ) containing mutations in both Buchnera genomes with more extensive
degradation in one genome. [Data from Tamas, I., Klasson, L., Canback, B.,
Naslund, A.K., Eriksson, A.-S., Wernegreen, J.J., Sandstrm, J.P., Moran, N.A.,
and Andersson, S.G.E. (2002). Science 296: 2376Ï2379 and Shigenobu, S.,
Watanabe, H., Hattori, M., Sakaki, Y., and Ishikawa, H. (2000). Nature 407:
81Ï86.]
homologous genes in Buchnera (Ap) are not affected. This may reÞect the particular interaction
of Buchnera (Sg) with its host. As reported elsewhere (Sandstrm et al., 1999), some aphid
species inÞict distinctive types of chlorotic lesions on their host plants, elevating the concen-
tration of amino acids such as cysteine in the phloem sap. This phenomenon may be related
to the observed silencing of genes for tryptophan biosynthesis in some lineages. On the
assumption that these are novel behaviors in evolutionary terms, it has been interpreted as a
decay of the mutualistic potential (Wernegreen and Moran, 2000).
C ELL -E NVELOPE G ENES
The accumulation of mutations in cell-envelope genes (murCEF, mraY, mltE, ddlB, nlpD, and mrcB )
is most likely related to the status of intracellular symbionts residing in the host-derived vesicles.
The mutations identiÝed in these genes are mainly single-nucleotide deletions and insertions in
homopolymeric tracts. Their effects on cell membrane structures remain to be determined.
DNA R EPAIR -A SSOCIATED G ENES
Genes coding for proteins involved in recombination/repair seem to have been targeted by mutations
to a larger extent than other genes. These mutated genes include endA , lig , ung , mfd, and phrB .
The functional inactivation of these genes may be related to the presence of, on average, 120
genomic copies per Buchnera cell.
 
 
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