Sinorhizobium meliloti RpoH1 is required for effective nitrogen-fixing symbiosis with alfalfa
Sinorhizobium meliloti RpoH1 is required for effective nitrogen-fixing symbiosis with alfalfa
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DOI：
10.1007/s00438-004-0992-x
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发表时间：
2004-03
影响因子：
3.1
通讯作者：
H. Mitsui;T. Sato;Y. Sato;N. Ito;K. Minamisawa
中科院分区：
文献类型：
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作者：
H. Mitsui;T. Sato;Y. Sato;N. Ito;K. Minamisawa
甜菜中华根瘤菌是一种根瘤固氮细菌。答案是。一株编码类似σ32蛋白的therpoH1基因的突变株在寄主植物紫花苜蓿上引起无效根瘤的形成。我们对与共生相关的表型进行了鉴定。与野生型相比，therpoH1突变体形成的紫花苜蓿根瘤的乙炔还原活性显著降低。虽然根瘤菌在顶端分生组织正下方的区域内观察到了细胞内的定植，但我们在近邻区域的许多寄主细胞中发现了超微结构异常和类杆菌的退化迹象。在结节的近端，只有少数结节细胞含有类杆菌。相反，在微氧培养中，therpoH1突变体表现出正常的固氮基因表达诱导。这些结果表明，热H1突变导致细菌在内生共生过程中提前衰老，但不影响入侵过程或固氮酶机制的合成。TherpoH1突变体对各种试剂和酸性pH表现出更高的敏感性，这表明RpoH1是保护细菌细胞免受宿主内环境压力的必需的。由于RpoH1以前被报道是合成一些热休克蛋白(HSP)所必需的，我们研究了几个HSP同源物基因的转录。我们发现，热休克后groESL5、Lon和clpB的转录依赖于RpoH1，并且在这些基因转录起始点上游的-35和-10区域发现了保守的核苷酸序列。虽然ESL5的表达几乎完全依赖于RpoH1，但我们发现农业ESL5突变株仍然能够在紫花苜蓿上正常共生固氮。
Sinorhizobium melilotiis a root-nodulating, nitrogen-fixing bacterium. AnS. melilotistrain that is mutant for therpoH1gene, which encodes a σ32-like protein, elicits the formation of ineffective nodules on the host plant alfalfa. We characterized therpoH1mutant for phenotypes related to symbiosis. Alfalfa nodules formed by therpoH1mutant exhibited greatly reduced levels of acetylene reduction activity compared to the wild-type nodules. Whereas intracellular colonization by rhizobia was observed in a zone just below the apical meristem, we found ultrastructural abnormalities and signs of degeneration of bacteroids within many host cells in the proximally adjacent zone. In the proximal part of the nodule, only a few nodule cells contained bacteroids. In contrast, therpoH1mutant showed normal induction of nitrogen fixation gene expression in microaerobic culture. These results suggest that therpoH1mutation causes early senescence of bacteroids during the endosymbiotic process, but does not affect the invasion process or the synthesis of the nitrogenase machinery. TherpoH1mutant exhibited increased sensitivity to various agents and to acid pH, suggesting that RpoH1is required to protect the bacterial cell against environmental stresses encountered within the host. Since RpoH1was previously reported to be required for the synthesis of some heat shock proteins (Hsps), we examined the transcription of several genes for Hsp homologs. We found that transcription ofgroESL5,lon, andclpBafter heat shock was RpoH1-dependent, and conserved nucleotide sequences were found in the –35 and –10 regions upstream of the transcription start sites of these genes. AlthoughgroESL5expression is almost completely dependent on RpoH1, we found that agroESL5mutant strain is still capable of normal symbiotic nitrogen fixation on alfalfa.