Larsen L K, Andreasen P H, Dreisig H, Palm L, Nielsen H, Engberg J, Kristiansen K
Department of Molecular Biology, Odense University, Campusvej 55, Odense M, DK-5260, Denmark.
Cell Biol Int. 1999;23(8):551-60. doi: 10.1006/cbir.1999.0419.
We have cloned and characterized the cDNA and the macronuclear genomic copy of the highly conserved ribosomal protein (r-protein) L3 of Tetrahymena thermophila. The r-protein L3 is encoded by a single copy gene interrupted by one intron. The organization of the promoter region exhibits features characteristic of ribosomal protein genes in Tetrahymena. The codon usage of the L3 gene is highly biased. A thorough analysis of codon usage in Tetrahymena genes revealed that genes could be categorized into two classes according to codon usage bias. Class A comprises r-protein genes and a number of other highly expressed genes. Class B comprises weakly expressed genes such as the conjugation induced CnjB and CnjC genes, but surprisingly, this class also contains abundantly expressed genes such as the genes encoding the surface antigens SerH3 and SerH1. Codon usage is slightly more restricted in class A than in class B, but both classes exhibit distinct and different codon usage biases. Class A genes preferentially use C and U in the silent third codon positions, whereas class B genes preferentially use A and U in the silent third codon positions. The analysis suggests that two different strategies have been employed for optimization of codon usage in the A+T-rich genome of Tetrahymena.
我们已克隆并鉴定了嗜热四膜虫高度保守的核糖体蛋白（r蛋白）L3的cDNA和大核基因组拷贝。r蛋白L3由一个被一个内含子打断的单拷贝基因编码。启动子区域的组织呈现出嗜热四膜虫核糖体蛋白基因的特征。L3基因的密码子使用存在高度偏好性。对嗜热四膜虫基因密码子使用的全面分析表明，基因可根据密码子使用偏好分为两类。A类包括r蛋白基因和一些其他高表达基因。B类包括弱表达基因，如接合诱导的CnjB和CnjC基因，但令人惊讶的是，这类还包含大量表达的基因，如编码表面抗原SerH3和SerH1的基因。A类基因的密码子使用比B类略受限制，但两类都表现出明显不同的密码子使用偏好。A类基因在沉默的第三个密码子位置优先使用C和U，而B类基因在沉默的第三个密码子位置优先使用A和U。该分析表明，在嗜热四膜虫富含A+T的基因组中，采用了两种不同的策略来优化密码子使用。
