Identification and analysis of micro-exons in AP2/ERF and MADS gene families.

Micro-exons are a set of ultrashort exons with lengths ≤ 51 nucleotides. Our previous study revealed that micro-exons were enriched in AP2 domains and K-box domains, which are crucial components of AP2/ERF (APETALA2/ethylene-responsive element-binding protein) and MADS-box (an acronym of MCM1, AGAMOUS, DEFICIENS and SRF) genes, respectively. In this study, we analyzed micro-exons in the AP2/ERF family from 63 species and demonstrated that 76.8% of micro-exons are concentrated in AP2 domains. Most micro-exons appeared in the AP2 subfamily of all the terrestrial plants, but not algae. In addition, micro-exons and AP2 domains are conserved and under negative selection. The MIKC gene is a typical MADS-box gene family in terrestrial plants and includes one MADS-box domain and one K-box domain. A total of 92.3% of micro-exons were observed in K-box domains, and two micro-exons usually encoded a region of K-box domain, which is the key to MADS-box protein polymerization. Furthermore, the micro-exons of the K-box domain had higher ratios of nonsynonymous mutations than those of the AP2 domains. Overall, here we explored the relationships and differences among micro-exons in AP2/ERF and MADS families, and revealed potential functional roles of micro-exons in these domains.