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2014 ; 12
(ä): 36
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Casposons: a new superfamily of self-synthesizing DNA transposons at the origin
of prokaryotic CRISPR-Cas immunity
#MMPMID24884953
Krupovic M
; Makarova KS
; Forterre P
; Prangishvili D
; Koonin EV
BMC Biol
2014[May]; 12
(ä): 36
PMID24884953
show ga
BACKGROUND: Diverse transposable elements are abundant in genomes of cellular
organisms from all three domains of life. Although transposons are often regarded
as junk DNA, a growing body of evidence indicates that they are behind some of
the major evolutionary innovations. With the growth in the number and diversity
of sequenced genomes, previously unnoticed mobile elements continue to be
discovered. RESULTS: We describe a new superfamily of archaeal and bacterial
mobile elements which we denote casposons because they encode Cas1 endonuclease,
a key enzyme of the CRISPR-Cas adaptive immunity systems of archaea and bacteria.
The casposons share several features with self-synthesizing eukaryotic DNA
transposons of the Polinton/Maverick class, including terminal inverted repeats
and genes for B family DNA polymerases. However, unlike any other known mobile
elements, the casposons are predicted to rely on Cas1 for integration and
excision, via a mechanism similar to the integration of new spacers into CRISPR
loci. We identify three distinct families of casposons that differ in their gene
repertoires and evolutionary provenance of the DNA polymerases. Deep branching of
the casposon-encoded endonuclease in the Cas1 phylogeny suggests that casposons
played a pivotal role in the emergence of CRISPR-Cas immunity. CONCLUSIONS: The
casposons are a novel superfamily of mobile elements, the first family of
putative self-synthesizing transposons discovered in prokaryotes. The likely
contribution of capsosons to the evolution of CRISPR-Cas parallels the
involvement of the RAG1 transposase in vertebrate immunoglobulin gene
rearrangement, suggesting that recruitment of endonucleases from mobile elements
as ready-made tools for genome manipulation is a general route of evolution of
adaptive immunity.