蒺藜苜蓿叶绿体密码子偏好性分析

摘要

The complete nucleotide sequence of the chloroplast genome of Medicago truncatula was investiga-ted.Fifty CDS (coding DNA sequences)selected from the chloroplast genome sequence of M.truncatula, were analyzed using CodonW software.The results show that the third codon position was rich in A and U. ENC ranged from 37.1 to 51.9 meaning that the codon bias was weak.There were 23 codons with relative syn-onymous codon usage greater than 1 and 20 codons ending with A and T.ENC-plot analysis showed that GC3 was not correlated with GC12 ;ENC ratio’s of most genes ranged from -0.05 to 0.05.In the correspondence analysis of the first group of four axes,the first axis showed 10.3% variation.The correlation coefficients for axis 1 between ENC and GC3 were 0.091 and -0.092 respectively (not significant).Synonymous codon usage bias was found,mainly due to the effect of mutation pressure,but there were other factors.In addition,analy-sis of the high expression codons enabled 23 to be affirmed as the “optimal codons”as UAA,UUG,CCU.The results provide evidence for molecular modification of exogenous genes to increase the expression efficiency in M.truncatula chloroplasts.%本文对蒺藜苜蓿叶绿体基因组全序列密码子进行分析，筛选出50条 CDS（coding DNA sequence）利用 Codo-nW 软件进行分析其密码子使用模式。结果显示，蒺藜苜蓿叶绿体基因组密码子第3位碱基 GC 含量为26．9％，即第3位密码子富含 A 和 U，ENC 值在37．11～51．91之间密码子偏好性较弱。相对同义密码子使用度分析显示RSCU 值大于1的密码子有23个，其中以 A 和 U 为结尾20个。中性绘图分析显示 GC12与 GC3的相关系数为0．341，相关性不显著，回归系数为0．4843；单基因 ENC 比值多分布在－0．05～0．05，即大部分基因 ENC 值离 ENC期望值较近；对应性分析，第一轴显示了12．50％的差异为主要影响因素，第一轴与 ENC 和 GC3的相关系数分别为0．091和－0．092，均相关不显著。综合这几项分析发现蒺藜苜蓿叶绿体基因组密码子偏好性主要受到突变的影响，但是并不是唯一的影响因素，其他因素对密码子偏好性也可能有一定的影响。最终通过高表达优越密码子方法确定得出 UUA、UUG、CCU 等23个密码子为最优密码子，为之后对外源基因进行改造，提高其在叶绿体中的表达效率奠定了基础。