Institut für Genetik der Universität zu Köln 5
Köln 41, Weyertal 121
The first extrachromosomal genetic elements discovered in bacteria
were temperate bacteriophages, which can exist either as extracellular
virus, as chromosomes replicating autonomously within cells or integrated
into the bacterial chromosome. Integration of viruses into host
chromosomes has subsequently attracted considerable attention by
tumor virologists. This may justify the discussion of other types
of genetic elements in bacteria in a meeting on human leucaemia.
In addition to temperate bacteriophages, there are other extrachromosomal
genetic elements in bacteria. Some of them are called plasmids.
They lack the extracellular state, but exist either autonomously
within the bacterial cytoplasm or integrated into the bacterial
chromosome. A third class of genetic elements has been described
in recent years. It apparently lacks the ability to multiply autonomously
and exists only integrated into the bacterial chromosome. Within
this chromosome, however, it can be transposed to various positions.
At present, three such elements, called IS 1, ( = insertion sequence
) IS2 and IS3 are known. Their length is 0.8, 1.4 and 1.25 kilobases,
respectively (1-5). The presence of these elements can be detected
by various effects which they show at the point of insertion:
1) Integration into the continuity of the gene causes the loss of
gene function (6,7).
2) Insertion into a gene within an operon causes severe polar effects
on genes distal to the mutated gene (6,7,8).
3) At least one representative of class IS2 carries a promoter.
Genes linked to this promoter are expressed constitutively (9).
4) If two circular chromosomes, e. g. the E. coli chromosome and
the F-factor , share an IS-element, e. g. IS2 or IS3, recombination
within the IS-elements leads to the fusion of these two chromosomes.
This mechanism accounts, at least in some instances, for the integration
of the F-factor into the E. coli chromosome upon formation of Hfr
strains or for the joining and disjoining of parts of bacterial
R -factors ( 10, 11) .
5) Two copies of the same IS-element, inserted in inverted position
relative to each other and bordering between them a certain gene
may form a transposon (12). The gene for tetracyclin resistance,
carried on plasmid R6-5 and also on phage P22 may serve as an example,
in which the IS-element is IS3 (13, 14).
The physical and biochemical characterization of IS-elements has
been possible in bacteria, due to the relative ease with which bacterial
and especially bacteriophage DNA can be handled. However, elements
with very similar properties have been characterized by genetic
methods in higher organisms also. Most notable are the controlling
elements in maize and mutable genes in Drosophila (15, 16). Should
it turn out that similar elements have a more widespread occurrence,
even among vertebrates, they may well deserve also the attention
of those interested in the genesis of malignant tumors.
Work done in the author's laboratory was supported by the Deutsche
through Sonderforschungsbereich 74.
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