Smooker P M, Kruft V, Subramanian A R
Max-Planck-Institut für Molekulare Genetik, Abteilung Wittmann, Berlin 33, Federal Republic of Germany.
J Biol Chem. 1990 Sep 25;265(27):16699-703.
The distribution of chloroplast ribosomal protein genes between the organelle DNA and the nuclear DNA is highly conserved in land plants, but a notable exception is rpl21. This gene has been found in the completely sequenced chloroplast genome of a lower plant but not in that of two higher plants. We describe the purification and characterization of the spinach chloroplast ribosomal protein L21 and the isolation and nucleotide sequence of a cDNA clone that encodes its cytoplasmic precursor. The mature protein, identified by NH2-terminal sequencing, has 201 residues (Mr 22,766) and is thus substantially larger than either its Escherichia coli (103 residues) or the lower plant homologue (116 residues). The extra length is in peptide extensions at both amino and carboxyl termini. The COOH-terminal extension is unusual in that it comprises seven Ala-Glu repeats, a feature not found in any other ribosomal proteins described so far. The cDNA clone also encodes a 55-residue long transit peptide (with a high proportion of the polar residues, threonine and serine), to target the L21 protein into chloroplasts. The identification of rpl21 as a nuclear gene in a higher plant (spinach) and chloroplast gene in a lower plant (liverwort) suggests an organelle-to-nucleus gene relocation during the evolution of the former.
叶绿体核糖体蛋白基因在细胞器DNA和核DNA之间的分布在陆地植物中高度保守，但rpl21是一个显著的例外。该基因已在一种低等植物的完全测序叶绿体基因组中发现，但在两种高等植物的叶绿体基因组中未发现。我们描述了菠菜叶绿体核糖体蛋白L21的纯化和特性，以及编码其细胞质前体的cDNA克隆的分离和核苷酸序列。通过氨基末端测序鉴定的成熟蛋白有201个氨基酸残基（Mr 22,766），因此比其大肠杆菌同源物（103个氨基酸残基）或低等植物同源物（116个氨基酸残基）大得多。额外的长度存在于氨基末端和羧基末端的肽段延伸中。羧基末端延伸部分不同寻常，因为它包含七个丙氨酸-谷氨酸重复序列，这一特征在目前描述的任何其他核糖体蛋白中都未发现。该cDNA克隆还编码一个55个氨基酸残基长的转运肽（极性残基苏氨酸和丝氨酸比例很高），用于将L21蛋白靶向导入叶绿体。rpl21在高等植物（菠菜）中作为核基因，而在低等植物（地钱）中作为叶绿体基因，这表明在前者的进化过程中发生了细胞器到细胞核的基因转移。
