中文摘要：

遗传密码子是生命信息的基本遗传单位,每种氨基酸对应1～6个同义密码子.特定物种在长期进化中形成了适应自身基因组环境的密码子使用偏性.运用CHIPS、CUSP和CodonW程序分析自主克隆的籽粒苋NAD-ME基因的密码子偏好性,并与马铃薯等7种植物的ME基因密码子偏好性进行比较,以期为该基因在作物遗传改良中选择合适的受体植物提供依据.结果表明,籽粒苋NAD-ME基因偏好于以A或T结尾的密码子,其它几种被比较作物的ME基因也有同样的趋势,但双子叶植物的偏好性更强.基于NAD-ME基因的密码子使用偏性的系统聚类分析表明,籽粒苋与马铃薯、拟南芥、葡萄、蓖麻、毛果杨等双子叶植物聚为1类,玉米和高粱这2个单子叶植物聚为1类,预示籽粒苋NAD-ME基因更适合导入马铃薯等双子叶植物.对籽粒苋NAD-ME基因的密码子偏好性与大肠杆菌和酵母的基因组密码子偏好性进行比较,发现均存在差异,与大肠杆菌的差异高于酵母,表明酵母表达系统要优于大肠杆菌表达系统.若要进一步提高籽粒苋NAD-ME基因在大肠杆菌或酵母中的表达水平,尚需对其密码子进行优化.

英文摘要：

Due to the degeneracy of genetic codon,most amino acids are coded by more than one codon（synonymous codons）.Nucleotide coding sequences of many organisms exhibit significant codon bias,that is,unequal usage of synonymous codons.In this paper,coding sequence of NAD-dependent malic enzyme（NAD-ME） gene of Amaranthus hypochondriacus was analyzed by Codon W,CHIPS（Condon heterozygosity in a protein coding sequence） and CUSP（Create a codon usage table） programs for identifying codon bias and selecting appropriate expression systems.The results showed that NAD-ME gene of A.hypochondriacus was bias toward the synonymous codons with A and T at the third codon position.The phylogenic analysis suggested that NAD-ME gene of A.hypochondriacus was evolutionarily closer to dicotyledons（Arabidopsis thaliana,Solanum,tuberosum,Vitis vinifera,Ricinus communis and Populus trichocarpa） than to monocotyledons（Zea mays and Sorghum bicolor）.There are 24 codons showing distinct usage differences between NAD-ME gene of A.hypochondriacus and genome of Escherichia coli,17 between NAD-ME gene of A.hypochondriacus and genome of yeast.Therefore,the yeast expression system may be more suitable for the expression of NAD-ME gene of A.hypochondriacus.