真核藻类WRKY基因家族的鉴定及表达分析

Identification and expression analysis of WRKY gene family in eukaryotic algae

摘要:

WRKY是植物特有的一个转录因子超家族，参与植物生长发育、物质代谢以及生物和非生物胁迫响应等多种生物学过程的调控。尽管WRKY转录因子基因已在高等植物广泛表征，然而有关真核藻类WRKY却知之甚少。采用多序列比对、系统进化和保守域分析等技术对30种真核藻类WRKY进行全基因组鉴定，共获得24个WRKY成员，均来自绿藻门（Chlorophyta）藻类。红藻门（Rhodophyta）、灰藻门（Glaucophyta）和硅藻门（Bacillariophyta）等藻类未检测到WRKY。24个WRKY成员均具有保守结构域七肽序列WRKYGQ （E/A/H/N） K和锌指基序C-X4-5-C-X22-23-H-X-H，分别归类于Ⅰ、Ⅱa、Ⅱb和R组。高含虾青素的雨生红球藻（Haematococcus pluvialis）含有2个Ⅰ类WRKY成员（HaeWRKY-1和HaeWRKY-2）。进一步克隆HaeWRKY-1和HaeWRKY-2基因编码序列，构建原核表达载体在大肠杆菌BL21（DE3）中诱导表达，并通过Ni-NTA亲和层析获得纯化的HaeWRKY融合蛋白。雨生红球藻在正常培养条件下，HaeWRKY-1表达量显著高于HaeWRKY-2。高光逆境胁迫显著上调HaeWRKY-1表达和下调HaeWRKY-2表达。HaeWRKY基因启动子含有多个光、乙烯、脱落酸（abscisic acid，ABA）以及逆境响应顺式元件。特别在HaeWRKY-2启动子区未检测到W-box顺式元件，但HaeWRKY-1和控制虾青素生物合成关键酶基因HaeBKT、HaePSY基因启动子含有W-box元件。基于本研究及前人的发现，我们推测高光胁迫下HaeWRKY-2低表达可能导致HaeWRKY-1的上调表达，HaeWRKY-1进而上调虾青素合成关键基因（HaeBKT、HaePSY等）表达，促进虾青素合成积累。这为深入解析雨生红球藻高光胁迫响应及虾青素合成的调控机制提供了新思路。

Abstract:

WRKY is a superfamily of plant-specific transcription factors, playing a critical regulatory role in multiple biological processes such as plant growth and development, metabolism, and responses to biotic and abiotic stresses. Although WRKY genes have been characterized in a variety of higher plants, little is known about them in eukaryotic algae, which are close to higher plants in evolution. To fully characterize algal WRKY family members, we carried out multiple sequence alignment, phylogenetic analysis, and conserved domain prediction to identify the WRKY genes in the genomes of 30 algal species. A total of 24 WRKY members were identified in Chlorophyta, whereas no WRKY member was detected in Rhodophyta, Glaucophyta, or Bacillariophyta. The 24 WRKY members were classified into Ⅰ, Ⅱa, Ⅱb and R groups, with a conserved heptapeptide domain WRKYGQ(E/A/H/N)K and a zinc finger motif C-X4-5-C-X22-23-H-X-H. Haematococcus pluvialis, a high producer of natural astaxanthin, contained two WRKY members (HaeWRKY-1 and HaeWRKY-2). Furthermore, the coding sequences of HaeWRKY-1and HaeWRKY-2 genes were cloned and then inserted into prokaryotic expression vector. The recombinant vectors were induced to express in Escherichia coli BL21(DE3) cells and the fusion proteins were purified by Ni-NTA affinity chromatography. HaeWRKY-1 had significantly higher expression level than HaeWRKY-2 in H. pluvialis cultured under normal conditions. High light stress significantly up-regulated the expression of HaeWRKY-1while down-regulated that of HaeWRKY-2. The promoters of HaeWRKY genes contained multiple cis-elements responsive to light, ethylene, ABA, and stresses. Particularly, the promoter of HaeWRKY-2 contained no W-box specific for WRKY binding. However, the W-box was detected in the promoters of HaeWRKY-1 and the key enzyme genes HaeBKT (β-carotene ketolase) and HaePSY (phytoene synthase) responsible for astaxanthin biosynthesis. Considering these findings and the research progress in the related fields, we hypothesized that the low expression of HaeWRKY-2 under high light stress may lead to the up-regulation of HaeWRKY-1 expression. HaeWRKY-1 may then up-regulate the expression of the key genes (HaeBKT, HaePSY, etc.) for astaxanthin biosynthesis, consequently promoting astaxanthin enrichment in algal cells. The findings provide new insights into further analysis of the regulatory mechanism of astaxanthin biosynthesis and high light stress response of H. pluvialis.