3.2. Non-templated additional nucleotides before
polyadenylation
Interestingly, we found that some genes tend to add some nucleotides
just before the polyadenylation sequence. Remarkably, this bias was
determined in both sense and antisense transcripts of Apis . We
found that 18.70% of all mapped transcripts (14,248 sequences) contain
at least one and at most seven additional nucleotides before
polyadenyls. 42.25% of the additional nucleotides were C, 30.28% G,
18.28% T and 9.19% A nucleotide (Figure 4g). It is suggested that
additional nucleotides before polyadenyls may be functional in the
processing of 3‘ ends in maize mitochondrial rps12 , cox2and atp9 genes (Williams, Johzuka and Mulligan, 2000).
Non-templated additional nucleotides were also found to be present in
human U2 snRNA and in some of Arabidopsis nuclear mRNAs in other
studies (Cho et al. , 2002; Jin and Bian, 2004). The additional
nucleotide detected in these three species mentioned above was
nucleotide C, and it was assumed that this nucleotide determines the
heterogeneity in transcript abundance (Jin and Bian, 2004). RNA-editing
mechanism is prevalent and mainly sustained with C to U editing in plant
mitochondrial transcription (Wu et al. , 2015). While it is
assumed that animal mitochondrial transcription system shorn of
RNA-editing mechanism, this additional nucleotide sites may be templates
of post-transcriptional regulations such as RNA-editing (Lynch, Koskella
and Schaack, 2006).