Chun J Y, Sexton G L, Roth L E, Stacey G
Center for Legume Research, Graduate Program of Plant Physiology and Genetics, Department of Zoology, and Department of Microbiology and Graduate Program of Ecology, University of Tennessee, Knoxville 37996.
J Bacteriol. 1994 Nov;176(21):6717-29. doi: 10.1128/jb.176.21.6717-6729.1994.
To understand the genetic mechanism of host specificity in the interaction between rhizobia and their hosts, it is important to identify genes that influence both early and late steps in symbiotic development. This paper focuses on the little-understood genetics of host-specific nitrogen fixation. A deletion mutant of Bradyrhizobium japonicum, strain NAD163, was found to induce effective, nitrogen-fixing nodules on soybean and siratro plants but produced ineffective nodules on cowpea plants. Additional transposon and deletion mutants defined a small region that conferred this phenotype, and this region was sequenced to identify two putative open reading frames (ORFs). Data indicate that only one of these ORFs is detectable in bacteroids. This ORF was termed hsfA, with a predicted protein product of 11 kDa. The transcriptional start site of hsfA was determined and found to coincide with a predicted RpoN-dependent promoter. Microscopic studies of nodules induced by the wild type and hsfA mutants on cowpea and soybean plants indicate that the cowpea mutant nodules are slow to develop. The data indicate that hsfA appears to play a crucial role in bacteroid development on cowpea but does not appear to be essential for nitrogen fixation on the other hosts tested.
为了理解根瘤菌与其宿主相互作用中宿主特异性的遗传机制，识别影响共生发育早期和晚期步骤的基因非常重要。本文聚焦于人们了解较少的宿主特异性固氮遗传学。发现日本慢生根瘤菌菌株NAD163的一个缺失突变体在大豆和西链豆植物上诱导形成有效的固氮根瘤，但在豇豆植物上产生无效根瘤。额外的转座子和缺失突变体确定了赋予这种表型的一个小区域，并对该区域进行测序以鉴定两个假定的开放阅读框（ORF）。数据表明在类菌体中仅可检测到这些ORF中的一个。这个ORF被命名为hsfA，预测的蛋白质产物为11 kDa。确定了hsfA的转录起始位点，发现其与预测的依赖于RpoN的启动子一致。对野生型和hsfA突变体在豇豆和大豆植物上诱导形成的根瘤进行显微镜研究表明，豇豆突变体根瘤发育缓慢。数据表明hsfA似乎在豇豆的类菌体发育中起关键作用，但对于所测试的其他宿主的固氮似乎并非必不可少。
