Jackman D M, Mulligan M E
Department of Biochemistry, Memorial University of Newfoundland, St John's, Newfoundland, Canada.
Microbiology (Reading). 1995 Sep;141 ( Pt 9):2235-44. doi: 10.1099/13500872-141-9-2235.
The exact identity of cyanobacteria that have been cultured from symbiotic associations with the water fern Azolla spp., whether they are required in the symbiotic process, and their relationship to the symbiotic species, is a matter of some debate. We have characterized a 6 kb region containing the nifB operon and the nifH gene from cyanobacterium Anabaena azollae 1a, a putative symbiont of Azolla caroliniana. Five complete open reading frames have been sequenced. All are very highly conserved when compared with the corresponding regions of Anabaena sp. PCC 7120, with 93% to 97% identity at the nucleotide level and 93% to almost 100% at the amino-acid level. The intergenic regions, however, are not highly conserved (53-89% identity) when compared to the corresponding regions of Anabaena 7120: the A. azollae genome contains both more copies and more types of short tandemly repeated repetitive sequences than Anabaena 7120. The start points of transcription for both the nifB and nifH operons were mapped and found to be the same as those in Anabaena 7120. It was not possible to discern an improved consensus nif promoter sequence, but it was possible to define the likely extent of the promoter to within 40 bases upstream of the transcription start-point.
从与满江红属水生蕨类植物的共生关系中培养出的蓝细菌的确切身份、它们在共生过程中是否是必需的，以及它们与共生物种的关系，存在一些争议。我们对来自卡罗来纳满江红假定共生体满江红鱼腥藻1a的包含nifB操纵子和nifH基因的6 kb区域进行了特征分析。已对五个完整的开放阅读框进行了测序。与鱼腥藻属PCC 7120的相应区域相比，所有这些开放阅读框都高度保守，在核苷酸水平上具有93%至97%的同一性，在氨基酸水平上具有93%至几乎100%的同一性。然而，与鱼腥藻7120的相应区域相比，基因间区域并不高度保守（同一性为53 - 89%）：满江红鱼腥藻基因组包含比鱼腥藻7120更多的短串联重复序列拷贝和更多类型的短串联重复序列。已对nifB和nifH操纵子的转录起始点进行了定位，发现与鱼腥藻7120中的相同。虽然无法识别出改进的nif启动子共有序列，但有可能将启动子的可能范围确定在转录起始点上游40个碱基以内。
