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).