Aging and Sex, DNA Repair in
87
5.1
Sexual Communication in Bacteria,
Primarily for DNA Repair
The simplest organisms in which sexual
communication occurs are bacteria. Bac-
terial transformation is a form of sexual
interaction in bacteria and has been shown
in several bacterial species to involve sexual
communication. Transformation involves
the transfer of naked DNA from one
member of a bacterial population to an-
other through the surrounding medium.
Transformation occurs naturally in a wide
range of bacterial species. The bacteria that
take up transforming DNA ordinarily in-
corporate this DNA into their genomes
by recombination. Transformation is an
evolved, rather than an incidental, trait
since it results from a complex, energy-
requiring, developmental process. For a
bacterium to bind, take up, and recom-
bine exogenous DNA into its genome, it
must ±rst enter a special physiological
state referred to as
competence
.T
r
a
n
s
-
formation involves expression of genes
required for competence and for recom-
bination. In the completely sequenced
genome of
Haemophilus influenzae
,trans
-
formation is promoted by at least 15 genes
of the 1007 for which role assignments
can be made. In another bacterium,
Bacil-
lus subtilis
, about 40 genes necessary for
competence have been identi±ed.
Transformation in a number of bacterial
species depends on the production of an
extracellular factor (referred to as
compe-
tence factor
), which, upon release into the
surrounding medium, induces the com-
petent state in neighboring cells in the
population. These extracellular factors are
examples of pheromones. (Pheromones
are molecules released by an organism into
the external medium to influence other
individuals of the same species.) These
extracellular factors have been studied in
B. subtilis, Streptococcus pneumoniae,
and
B. cereus
.In
B. subtilis,
two different extra-
cellular peptide pheromones are used for
competence development. In
S. pneumo-
niae,
the competence pheromone is a 17
amino acid peptide.
The adaptive function of bacterial trans-
formation has been studied in
B. subtilis
as a model system for understanding
the adaptive function of bacterial trans-
formation generally. The results indicated
that transformation provides external DNA
template for recombinational repair of
DNA damage in the recipient. Trans-
formation and recombinational repair in
B. subtilis
require the RecE protein, which
is a homolog of the well-studied RecA
protein of
E. coli
.DNAd
am
a
g
e
sa
r
i
s
-
ing from such sources as UV irradiation,
endogenous oxidative free radicals, and
desiccation are a pervasive problem for
bacteria. The RecE protein of
B. subtilis
probably catalyzes recombinational repair
of these prevalent DNA damages. Thus,
sexual communication via pheromones in
bacteria is probably an evolved mechanism
to promote sexual interaction for recombi-
national repair of prevalent DNA damages.
This is ‘‘Level 1’’, the primary level of sex-
ual communication.
Transformation in bacteria involves par-
tial diploidy as an intermediate stage.
Even though the initial advantage of this
process, we think, was recombinational re-
pair of DNA damage, diploidy (or partial
diploidy) may also provide the bene±t of
masking deleterious recessive mutations
through complementation. Thus, recom-
binational repair in bacteria may have
set the stage, or may have been a pre-
cursor to, natural selection for diploidy
as a distinct phase of the sexual cycle
in eukaryotes. Further, recombinational
variation is produced as a by-product
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