De Feyter R, Yang Y, Gabriel D W
Plant Pathology Department, University of Florida, Gainesville 32611.
Mol Plant Microbe Interact. 1993 Mar-Apr;6(2):225-37. doi: 10.1094/mpmi-6-225.
Six plasmid-borne avirulence (avr) genes were previously cloned from strain XcmH of the cotton pathogen, Xanthomonas campestris pv. malvacearum. We have now localized all six avr genes on the cloned fragments by subcloning and Tn5-gusA insertional mutagenesis. None of these avr genes appeared to exhibit exclusively gene-for-gene patterns of interactions with cotton R genes, and avrB4 was demonstrated to confer avr gene-for-R genes (plural) avirulence to X. c. pv. malvacearum on congenic cotton lines carrying either of two different resistance loci, B1 or B4. Furthermore, the B1 locus appeared to confer R gene-for-avr genes resistance to cotton against isogenic X. c. pv. malvacearum strains carrying any one of three avr genes: avrB4, avrb6, or avrB102. Restriction enzyme, Southern blot hybridization, and DNA sequence analyses showed that the XcmH avr genes are all highly similar to each other, to avrBs3 and avrBsP from the pepper pathogen X. c. pv. vesicatoria, and to the host-specific virulence gene pthA from the citrus pathogen X. citri. The XcmH avr genes differed primarily in the multiplicity of a tandemly repeated 102-base pair motif within the central portions of the genes, repeated from 14 to 23 times in members of this gene family. The complete nucleotide sequence of avrb6 revealed that it is 97% identical in DNA sequence to avrB4, avrBs3, avrBsP, and pthA and that 62-bp inverted terminal repeats mark the boundaries of homology between avrb6 and all members of this Xanthomonas virulence/avirulence gene family sequenced to date. The terminal 38 bp of both inverted repeats are highly similar to the 38-bp consensus terminal sequence of the Tn3 family of transposons. Up to 11 members of the avr gene family appear to be present in North American strains of X. c. pv. malvacearum, including XcmH. The high level of homology observed among these avr genes and their presence in multiple copies may explain the gene-for-genes interactions and also the observed high frequencies (10(-3) to 10(-4) per locus) of X. c. pv. malvacearum race change mutations. Five spontaneous race change mutants of XcmH suffered avr locus deletions, strongly indicating intergenic recombination as the primary mechanism for generating new races in X. c. pv. malvacearum.
先前已从棉花病原菌野油菜黄单胞菌致病变种（Xanthomonas campestris pv. malvacearum）的XcmH菌株中克隆出6个质粒携带的无毒（avr）基因。我们现在通过亚克隆和Tn5-gusA插入诱变将所有6个avr基因定位在克隆片段上。这些avr基因似乎都没有表现出与棉花R基因完全的基因对基因相互作用模式，并且avrB4被证明能赋予野油菜黄单胞菌致病变种对携带两个不同抗性位点B1或B4之一的同基因棉花品系的avr基因对多个R基因的无毒力。此外，B1位点似乎赋予棉花对携带三个avr基因（avrB4、avrb6或avrB102）中任何一个的同基因野油菜黄单胞菌致病变种的R基因对avr基因的抗性。限制性内切酶、Southern印迹杂交和DNA序列分析表明，XcmH的avr基因彼此之间高度相似，与辣椒病原菌野油菜黄单胞菌致病变种（X. c. pv. vesicatoria）的avrBs3和avrBsP以及柑橘病原菌柑橘黄单胞菌（X. citri）的宿主特异性毒力基因pthA也高度相似。XcmH的avr基因主要区别在于基因中部串联重复的102个碱基对基序的重复次数，在这个基因家族的成员中重复14至23次。avrb6的完整核苷酸序列显示，其DNA序列与avrB4、avrBs3、avrBsP和pthA的同源性为97%，并且62个碱基对的反向末端重复序列标记了avrb6与该黄单胞菌毒力/无毒力基因家族迄今测序的所有成员之间的同源性边界。两个反向重复序列的末端38个碱基与转座子Tn3家族的38个碱基共有末端序列高度相似。在北美野油菜黄单胞菌致病变种的菌株中，包括XcmH，似乎存在多达11个avr基因家族成员。在这些avr基因中观察到的高度同源性及其多拷贝存在可能解释了基因对基因的相互作用，也解释了观察到的野油菜黄单胞菌致病变种小种变化突变的高频率（每个位点10^(-3)至10^(-4)）。XcmH的5个自发小种变化突变体发生了avr位点缺失，强烈表明基因间重组是野油菜黄单胞菌致病变种产生新小种的主要机制。
