  Mirtrons are a special type of pre-miRNA which originate from intronic
regions and are spliced directly from the transcript instead of being processed
by Drosha. The splicing mechanism is better understood for the processing of
mRNA for which was established that there is a characteristic CG content around
splice sites. Here we analyse the CG-content ratio of pre-miRNAs and mirtrons
and compare them with their genomic neighbourhood in an attempt to establish
key properties which are easy to evaluate and to understand their biogenesis.
We propose a simple log-ratio of the CG-content comparing the precursor
sequence and is flanking region. We discovered that Caenorhabditis elegans and
Drosophila melanogaster mirtrons, so far without exception, have smaller
CG-content than their genomic neighbourhood. This is markedly different from
usual pre-miRNAs which mostly have larger CG-content when compared to their
genomic neighbourhood. We also analysed some mammalian and primate mirtrons
which, in contrast the invertebrate mirtrons, have higher CG-content ratio.
