ANNOUNCEMENTS
Polyploidy has been a significant evolutionary process promoting plant genome evolution and functional diversification. Several studies have been initiated to comprehend the evolutionary consequences of polyploidy at genome wide level. However, influence of polyploidy on the cis-regulatory elements is poorly studied. The upstream regulatory elements regulate the gene expression patterns and levels. The present study aimed at sequence characterization of MIR160 proximal promoters and determining the evolutionary fate of MIR160 paralogs and homeologs in polyploid species of Brassica. MIR160, a highly conserved miRNA family, plays critical roles in plant development by negatively regulating the auxin response factors ARF10, ARF16 and ARF17. Across five Brassica genomes (B. juncea, B. rapa, B. nigra, B. oleracea, B. napus), 52 homologous copies of MIR160 have been found. The upstream regulatory sequences (2 kb) were retrieved and additionally, homologous sequences of A. thaliana were also isolated for comparative analysis. Examination of distribution and frequency of transcription factor binding sites (TFBS) present in homeologous proximal promoters, led to the identification of sub-genome specific and species-specific TFBS. Further analysis revealed that several TF-families were exclusively present in various Brassica species. Moreover, to analyse the extent of sequence conservation and divergence among promoters, comparative genomics approach was employed which revealed high degree of conservation towards the proximal side of promoters that progressively reduces towards the regions located further upstream. Presence of conserved regions suggested higher selective constraint for retention multiple copies of MIR160; plausibly the functional divergence is attributable to change in TFBS repertoire. Phylogenetic analysis and examination of presence and frequency of functional TFBSs in the promoters showed that homeologs were closely related. Through the present study, we suggest the relevance of unravelling the evolutionary dynamics and functional consequences of divergence in miRNA promoters in polyploids.
Keywords: Polyploidy, promoters, Brassica, MIR160, homeologs, TFBS.
