Department of Molecular Biology, Agricultural University, De Dreijen 11, 6703 BC Wageningen, The Netherlands.
Proc Natl Acad Sci U S A. 1987 Jul;84(13):4495-9. doi: 10.1073/pnas.84.13.4495.
Establishment of a nitrogen-fixing root nodule is accompanied by a developmentally regulated expression of nodulin genes, only some of which, the so-called early nodulin genes, are expressed in stages preceding actual nitrogen fixation. We have isolated soybean cDNA clones representing early nodulin genes and have studied clone pENOD2 in detail. The cDNA insert of this clone hybridizes to nodule-specific RNA of 1200 nucleotides in length. The RNA that was hybrid-selected by the cloned ENOD2 DNA was in vitro translated to produce two nodulins with an apparent M(r) of 75,000, the N-75 nodulins. These two nodulins differ slightly in charge and one does not contain methionine. The amino acid sequence deduced from the DNA sequence shows that proline accounts for 45% of the 240 residues in these nodulins and the sequence contains at least 20 repeating heptapeptide units. The amino acid composition of none of the (hydroxy)proline-rich (glyco)proteins described in plants resembles the composition of the N-75 nodulins, especially with respect to the high glutamic acid and the low serine content. This suggests that the N-75 nodulins belong to a hitherto unidentified class of presumably structural proteins. The genes encoding the N-75 nodulins were found to be expressed in nodule-like structures devoid of intracellular bacteria and infection threads, indicating that these nodulins do not function in the infection process but more likely function in nodule morphogenesis.
固氮根瘤的形成伴随着豆球蛋白基因的发育调控表达，只有其中一些，所谓的早期豆球蛋白基因，在实际固氮之前的阶段表达。我们已经分离出代表早期豆球蛋白基因的大豆 cDNA 克隆，并详细研究了克隆 pENOD2。该克隆的 cDNA 插入片段与 1200 个核苷酸长的特定于根瘤的 RNA 杂交。用克隆的 ENOD2 DNA 杂交选择的 RNA 在体外翻译产生两个表观分子量为 75000 的豆球蛋白，即 N-75 豆球蛋白。这两种豆球蛋白在电荷上略有不同，其中一种不含蛋氨酸。从 DNA 序列推导的氨基酸序列表明，脯氨酸占这些豆球蛋白中 240 个残基的 45%，并且该序列至少包含 20 个重复的七肽单元。植物中描述的（羟）脯氨酸丰富的（糖）蛋白没有一种的氨基酸组成与 N-75 豆球蛋白的组成相似，尤其是谷氨酸含量高而丝氨酸含量低。这表明 N-75 豆球蛋白属于迄今尚未鉴定的一类假定结构蛋白。发现编码 N-75 豆球蛋白的基因在缺乏细胞内细菌和感染线的类根瘤结构中表达，表明这些豆球蛋白不在感染过程中发挥作用，而更可能在根瘤形态发生中发挥作用。
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