Tanaka R, Yoshida K, Nakayashiki T, Masuda T, Tsuji H, Inokuchi H, Tanaka A
Department of Botany, Kyoto University, Japan.
Plant Physiol. 1996 Apr;110(4):1223-30. doi: 10.1104/pp.110.4.1223.
The first committed step of porphyrin synthesis in higher plants is the reduction of glutamyl-tRNA to glutamate 1-semialdehyde. This reaction is catalyzed by glutamyl-tRNA reductase, which is encoded by hemA genes. Two hemA cDNA clones (hemA1 and hemA2) were obtained from cucumber (Cucumis sativus) cotyledons by the PCR and cDNA library screening. They showed significant homology with published hemA sequences. Southern blot analysis of cucumber genomic DNA revealed that these genes are located at different loci and that there is another gene similar to the hemA genes. Accumulation of hemA1 mRNA was detected primarily in cotyledons and hypocotyls of greening cucumber seedlings, whereas that of hemA2 mRNA was detected in all tissues examined. Illumination of cucumber seedlings increased markedly the accumulation of hemA1 mRNA, but it did not induce remarkable changes in that of hemA2 mRNA. These findings suggest that hemA1 mRNA was accumulated in response to the demand of Chl synthesis in photosynthesizing tissues, whereas hemA2 mRNA was expressed in response to the demand of the synthesis of porphyrins other than chlorophylls.
高等植物中卟啉合成的第一个关键步骤是将谷氨酰 - tRNA还原为谷氨酸 - 1 - 半醛。此反应由谷氨酰 - tRNA还原酶催化，该酶由hemA基因编码。通过PCR和cDNA文库筛选从黄瓜（Cucumis sativus）子叶中获得了两个hemA cDNA克隆（hemA1和hemA2）。它们与已发表的hemA序列具有显著同源性。对黄瓜基因组DNA的Southern印迹分析表明，这些基因位于不同位点，并且存在另一个与hemA基因相似的基因。主要在绿化黄瓜幼苗的子叶和下胚轴中检测到hemA1 mRNA的积累，而在所有检测的组织中均检测到hemA2 mRNA的积累。黄瓜幼苗的光照显著增加了hemA1 mRNA的积累，但未诱导hemA2 mRNA发生显著变化。这些发现表明，hemA1 mRNA是响应光合组织中叶绿素合成的需求而积累的，而hemA2 mRNA是响应除叶绿素以外的卟啉合成需求而表达的。
