Comprehensive genomic survey， structural classification and expression analysis of C2H2 zinc finger protein gene family in Brassica rapa L.

C2H2 zinc finger protein (ZFP) genes have been extensively studied in many organisms and can function as transcription factors and be involved in many biological processes including plant growth and development and stress responses. In the current study， a comprehensive genomics analysis of the C2H2-ZFP genes in B. rapa was performed. A total of 301 B. rapa putative C2H2-ZFP (BrC2H2-ZFP) genes were identified from the available Brassica genome databases， and further characterized through analysis of conserved amino acid residues in C2H2-ZF domains and their organization， subcellular localization， phylogeny， additional domain， chromosomal location， synteny relationship， Ka/Ks ratio， and expression pattern. We also analyzed the expression patterns of eight B. rapa C2H2-ZFP genes under salt and drought stress conditions by using qRT-PCR technique. Our results showed that about one-third of these B. rapa C2H2-ZFP genes were originated from segmental duplication caused by the WGT around 13 to 17 MYA， one-third of them were highly and consecutively expressed in all tested tissues， and 92% of them were located in nucleus by prediction supporting then their functional roles as transcription factors， of which some may play important roles in plant growth and development. The Ka/Ks ratios of 264 orthologous C2H2-ZFP gene pairs between A. thaliana and B. rapa were all， except two， inferior to 1 (varied from 0.0116 to 1.4919， with an average value of 0.3082)， implying that these genes had mainly experienced purifying selection during species evolution. The estimated divergence times of the same set of gene pairs ranged from 6.23 to 38.60 MY， with an average value of 18.29 MY， indicating that these gene members have undergone different selective pressures resulting in different evolutionary rates during species evolution. In addition， a few of these B. rapa C2H2-ZFPs were shown to be involved in stress responses in a similar way as their orthologs in A. thaliana. Comparison between A. thaliana and B. rapa orthologous C2H2-ZFP genes showed that the majority of these C2H2-ZFP gene members encodes proteins with conserved subcellular localization and functional domains between the two species but differed in their expression patterns in five tissues or organs. Thus， our study provides valuable information for further functional determination of each C2H2-ZFP gene across the Brassica species， and may help to select the appropriate gene targets for further in-depth studies， and genetic engineering and improvement of Brassica crops.