Biology Reference
In-Depth Information
plasmid are immune to the action of the toxin. The best known bacteriocin is colicin, which is
found in many
E. coli
strains. The colicin-producing plasmid ColE1 is the ancestor of most plasmids
used in molecular biology. The mode of action of various colicins is quite different. Colicin E1
permealizes the cytoplasmic membrane, colicin E2 leads to the degradation of DNA, and colicin
E3 leads to the degradation of ribosomal RNA. Some bacteriocins like enterocoliticin have evolved
from bacteriophages. The toxins form high-molecular-weight phage tail-like structures that in some
cases still show contraction upon contact with susceptible bacteria. The R2 pyocin of
Pseudomonas
aeruginosa
is related to phage P2, and the F2 pyocin is related to phage lambda (Nakayama et al.,
2000; Strauch et al., 2001). The nature of the plasmid-encoded antitoxins is much less well under-
stood. Many lactic-acid bacteria harbor more than one bacteriocin system with often complementary
antimicrobial activities. Bacteriocins of both classes, initially synthesized as preproteins, are
secreted by a dedicated ABC transporter system that is also encoded by the bacteriocin cluster. A
few bacteriocins make use of the more general
sec
-dependent secretory system (Baba and Schnee-
wind, 1998; Riley, 1998; McAuliffe et al., 2001; OÔConnor and Shand, 2002).
Intracellular Microorganisms
The bacterial relE toxin can kill yeast and human cells, and its cognate bacterial antitoxin exhibits
rescue activity in eukaryotes (Kristoffersen et al., 2000; Yamamoto et al., 2002). CMS caused by
an organelle killing its eukaryotic host cell represents an independent system actively secreting
components of its addiction system, and it may indicate a paradigm of how a prokaryote (or a
symbiont) starts to manipulate its eukaryotic hosts. It also documents vividly the conÞict between
selÝsh systems in the cytoplasm and the nucleus. If seen from the outside, cytoplasmic inheritance
leads to intragenomic conÞict, which likely is the driving force behind uniparental cytoplasmic
inheritance (Cosmides and Tooby, 1981; Law and Hutson, 1992).
Not surprisingly, there are a wide and steadily increasing variety of microorganisms, and in
particular bacteria, that are cytoplasmic symbionts exerting drastic effects on their host cells
(
Chapter 12,
this topic; Braig et al., 2002). It seems to be evolutionarily easy to develop into a
selective killing system. This is best illustrated by the many bacteria that selectively kill males very
early on during embryogenesis (see Chapter 12). However, there has been so far only one bacterium
identiÝed whose host killing can manifest itself as CI. This bacterium is
W. pipientis
.
WOLBACHIA
-INDUCED CI
The bacterium
Wolbachia
was Ýrst described by Hertig and Wolbach as a rickettsial infection of
the gonads of
Culex pipiens
(Hertig and Wolbach, 1924; Hertig, 1936), but it was not until 1971
that Yen and Barr proposed that
Wolbachia
was the etiological factor for CI (Yen and Barr, 1971,
1973; Yen, 1972). CI in
C. pipiens
had already been observed in the late 1930s (Marshall and
Staley, 1937), and Laven had recognized its potential to control mosquitoes by 1951 (Laven, 1951).
By the late 1960s CI was employed as a vector-control strategy (Laven, 1967; Awahmukalah and
Brooks, 1983; Braig and Yan, 2002;
Chapter 13
of this topic).
W. pipientis
has a long and successful
relationship with arthropods and nematodes, collectively the ÑEcdyozoa.Ò Presumably, more than
15% of all known insect species carry it (Werren and OÔNeill, 1997; Bourtzis and Braig, 1999;
Stouthamer et al., 1999; Anderson and Karr, 2001; Stevens et al., 2001). Here we describe the CI
phenomenon and its distribution as well as the current knowledge about the mechanisms involved.
D
ESCRIPTION
The genetic phenomenon of CI was originally described in the mosquito
C. pipiens
when Laven
and Ghelelovitch conÝrmed the occurrence of reproductive isolation between different mosquito
populations (Laven, 1951, 1953; Ghelelovitch, 1952). Ghelelovitch showed that males from a
