Reference: Hall and Collis. 1995. Molec. Microbiol. 15:593-600.
Integron - genetic unit that includes genes of a site-specific
recombination system capable of capturing and mobilizing genes
contained in mobile elements called gene cassettes.
Integron structure consists of two
conserved regions (intI regions and sulI region)
flanking a variable region (gene cassette) which contains one
or more resistance genes. Cassette-free integrons exist in which
the int and sul regions are contiguous.
The intI region encodes integrase.
The sulI region confers resistance to sulphonamides.
The gene cassette is mobile and can
also exist in free circular form. However, these cassettes do
not include all functions required for their mobility. Cassettes
are only formally part of the integron when they are integrated
at the integron-receptor site. The genes within the cassette are
promoterless.
It is interesting to note that this is thus a site-specific recombination
system in which the integrase function is supplied by the recipient
(the integron) and not the mobile insertion unit (the gene cassette).
The inserted genes of the mobile cassette are all in the same
orientation with respect to the int (5'-CS [conserved sequence])
and sul (3'-CS) regions. This orientation allows them to
be transcribed from a common promoter, Pant,
that is located in the 5'-CS and is responsible for transcription
of the antibiotic-resistance genes.
When more than one cassette is present, the cassette-encoded genes
are co-transcribed from Pant. However,
downstream genes are transcribed less frequently due to premature
termination of transcripts.
The recombination site that acts as the receptor site for insertion
of the cassettes is a 40 to 70-base element called attI.
This site spans the 5'-CS and 3'-CS junction and the distal portions
lie within these conserved regions. The site within the cassette
that interacts with the attI site is called a 59-base element
which in a circular free cassette consists of two inverted repeats.
Loss of cassettes is dependent on integrase function and excision
results in circular free cassettes. These free cassettes are not
able to replicate but are able to participate in integration into
another integron, thus spreading the antibiotic resistance genes
from one integron to another.
The insertion/excision events mediated by the integron-encoded
integrase can lead to cointegrate formation between two integron-containing
plasmids present in the same cell and the subsequent resolution
of these cointegrates. If cointegrate resolution occurs at sites
other than those used for integration, a reciprocal
acquisition and loss of gene cassettes can occur resulting
in one cassette-free integron and one integron with two integrated
cassettes. This may be significant if the new recipient host plasmid
is transferrable or has a broader host range.
No information is as yet available on the origin of gene cassettes.
One hypothesis is that they arose from reverse transcription of
a mRNA, which would explain why gene cassettes contain genes with
no flanking sequences such as promoters. The 59-base element may
have originated from a transcription terminator (a hairpin loop)
and in fact recent evidence indicates that these elements can
indeed function as terminator sites.
URL: http://www.science.siu.edu/microbiology/micro460/460%20Pages/460.integrons.html
Last updated: 28-Oct-02 / laa