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The Spm (suppressor-mutator) elements are one kind of controlling element
(the same as transposon) families.
Moreover, the Spm elements belong to autonomous controlling elements
which encode proteins that enable them to excise and transpose.
A nonautonomous controlling element, dSpm (defective Spm),
are closely related to the Spm element.
The controlling elements can transpose in genome, when the DNA of chromosome
is hemimethylated (the typical state immediately after replication).
Therefore, the methylation is probably the major mechanism that is used
to prevent transposons from damaging the genome by transposing too frequently.
Fig 1. The model of Spm elements. The gene of tnpA transcripts 8300 bp of
pre-mRNA, which contains 11 exons and two open reading frames (ORFs) of ORF1
and ORF2. After processing alternative splicing, the 2500 bp mRNA is produced
and translated into Tnp A with 621 amino acids. The ORF1 and ORF2 can code
for Tnp B. Both of Tnp A and Tnp B can bind to the 13 bp terminal inverted
repeats of Spm elements to cleave for transposition.
Both of Spm and dSpm elements affect gene expression at their sites of
insertion.
For Spm elements
Spm-suppressible allele: To inhibit target gene expression
Suppression is caused by TnpA to bind its target site and then excise,
which blocks transcription.
Spm-dependent allele: To aid target gene expression
For dSpm elements
dSpm-suppressible allele: To inhibit target gene expression
The dSpm elements inserted within an exon of a gene can result in
frameshift to block transcription, the false product of RNA splicing,
and the incorrect mRNA which is produce from the excise the dSpm element
by TnpA and TnpB.
dSpm-dependent allele: To aid target gene expression
The dSpm elements inserted near a gene, not within a gene, can provide
an enhancer that activates the promoter of the gene at the recipient locus.
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