ISSN 1022-7954, Russian Journal of Genetics, 2006, Vol. 42, No. 11, pp. 1294–1302. © Pleiades Publishing, Inc., 2006.
Original Russian Text © T.S. Ilyina, 2006, published in Genetika, 2006, Vol. 42, No. 11, pp. 1536–1546.
1294
INTRODUCTION
The role of mobile genetic elements in the evolu-
tionary adaptation of microorganisms to the rapidly
changing environment is widely recognized. A signifi-
cant contribution to this process has been made by plas-
mids, transposons, and bacteriophages, which have
developed specific mechanisms of transfer between
cells of closely and remotely related organisms. The
horizontal spread of various genes among organisms
belonging to different groups was effected through the
agency of all of these elements.
The deciphering of nucleotide sequences in DNA of
bacterial, plasmid, and phage genomes, along with the
application of novel molecular biological technologies,
allowed the determination of previously unknown
mechanisms of bacterial evolution, and answered the
question as to how the sets of new genes encoding
adaptive functions emerge and what is the origin of
these genes. The greatest progress in this field has been
made when studying the mechanisms of the spread of
drug resistance genes among bacteria. This problem
attracts much interest, which is not unexpected,
because the increasingly large number of pathogenic
bacteria with multiple resistance is an urgent problem
of medical practice.
Study of drug resistance in bacteria led in the 1990s
to the discovery of a new type of mobile genetic ele-
ments, gene cassettes, and structures with a specific
organization, integrons, which can capture gene cas-
settes via their integration into specific integration sites
and express cassette-associated genes. In these years, a
fairly large number of experimental works and reviews
on integrons appeared [1–12]. However, many ques-
tions regarding the origin of gene cassettes and inte-
grons encoding multiple resistance have not yet been
answered. In addition, it was unclear whether other
genes, not connected with antibiotic resistance, have a
cassette organization.
Recently, new types of integrons (superintegrons)
were found, and their study provided answers to a num-
ber of the questions raised. In this paper, data on the
structural organization of the so-called “platform of
integrons,” gene cassettes, and integrons with inte-
grated gene cassettes encoding drug resistance will be
briefly summarized. Data obtained during recent years
in studies on superintegrons or chromosomal integrons,
the properties of their structural organization, the pres-
ence in them of genes with known adaptive or uniden-
tified functions, the data on differences between super-
integrons and multiple resistance integrons, as well as
evidence for the translocation of gene cassettes from
stationary chromosomal superintegrons into traditional
integrons, will be considered in more detail.
GENE CASSETTES
Gene cassettes are autonomous nonreplicating
covalently closed circular elements [4, 8, 10] contain-
ing one open reading frame and one recombination site
(ORF–attC elements) each. The original name of recom-
bination sites in gene cassettes was 59 be (59 base ele-
ments), because the size of the first site studied was
equal to 59 bp. Later, the size of these sites was shown
to vary within the range 57–141 bp in different cas-
settes. Therefore, the term attC was introduced to des-
ignate cassette recombination sites in subsequent pub-
lications of many authors.
Bacterial Superintegrons, a Source of New Genes
with Adaptive Functions
T. S. Ilyina
Gamaleya Research Institute of Epidemiology and Microbiology, Russian Academy of Medical Sciences,
Moscow, 123098 Russia; e-mail: jmromanova@riem.ru
Received January 23, 2006
Abstract
—Data on the structural organization of the platform of integrons, gene cassettes, and integrons with
integrated cassettes of genes encoding drug resistance are briefly summarized. Data obtained during recent
years about superintegrons or chromosomal integrons, characteristics of their organization, the presence of
genes with known adaptive and unidentified functions in them, as well as data on the differences between super-
integrons and previously described multiple resistance integrons, are considered in more detail. Studies that
provide evidence for translocations of gene cassettes from stationary chromosomal integrons into integrons
associated with mobile elements resulting in gene flows in natural bacterial populations, which favors their sur-
vival and adaptation to changing environment, are also reviewed.
DOI:
10.1134/S1022795406110111
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