Noormohammadi A H, Markham P F, Kanci A, Whithear K G, Browning G F
School of Veterinary Science, The University of Melbourne, Cnr. Flemington Road and Park Drive, Parkville, Victoria, Australia.
Mol Microbiol. 2000 Feb;35(4):911-23. doi: 10.1046/j.1365-2958.2000.01766.x.
High-frequency phase and antigenic variation of homologous lipoprotein haemagglutinins has been seen in both the major avian mycoplasma pathogens, Mycoplasma synoviae and Mycoplasma gallisepticum. The expression and, hence, antigenic variation of the pMGA gene family (encoding these lipoproteins in M. gallisepticum) is controlled by variation in the length of a trinucleotide repeat motif 5' to the promoter of each gene. However, such a mechanism was not detected in preliminary observations on M. synoviae. Thus, the basis for control of variation in the vlhA gene family (which encodes the homologous haemagglutinin in M. synoviae) was investigated to enable comparison with its homologue in M. gallisepticum and with other lipoprotein gene families in mycoplasmas. The start point of transcription was identified 119 bp upstream of the initiation codon, but features associated with control of transcription in other mycoplasma lipoprotein genes were not seen. Comparison of three copies of vlhA revealed considerable sequence divergence at the 3' end of the gene, but conservation of the 5' end. Southern blot analysis of M. synoviae genomic DNA revealed that the promoter region and part of the conserved 5' coding sequence occurred as a single copy, whereas the remainder of the coding sequence occurred as multiple copies. A 9.7 kb fragment of the genome was found to contain eight tandemly repeated regions partially homologous to vlhA, all lacking the putative promoter region and the single-copy 5' end of vlhA, but extending over one of four distinct overlapping regions of the 3' coding sequence. Examination of sequential clones of M. synoviae established that unidirectional recombination occurs between the pseudogenes and the expressed vlhA, with duplication of pseudogene sequence and loss of the corresponding region previously seen in the expressed gene. Expression of the 5' end of two variants of the vlhA gene showed that they differed in their reaction with monoclonal antibodies specific for this region. These data suggest that the control of vlhA antigenic variation in M. synoviae is achieved by multiple gene conversion events using a repertoire of coding sequences to generate a chimeric expressed gene, with the greatest potential for variation generated in the region encoding the haemagglutinin. Thus, completely distinct mechanisms have been adopted to control antigenic variation in homologous gene families.
在两种主要的禽支原体病原体——滑液支原体和鸡毒支原体中，均发现了同源脂蛋白血凝素的高频相位和抗原变异。鸡毒支原体中编码这些脂蛋白的pMGA基因家族的表达以及由此产生的抗原变异，受每个基因启动子5'端三核苷酸重复基序长度变化的控制。然而，在对滑液支原体的初步观察中未检测到这种机制。因此，研究了vlhA基因家族（在滑液支原体中编码同源血凝素）变异的控制基础，以便与鸡毒支原体中的同源物以及支原体中的其他脂蛋白基因家族进行比较。转录起始点在起始密码子上游119 bp处被确定，但未发现与其他支原体脂蛋白基因转录控制相关的特征。对三个vlhA拷贝的比较显示，该基因3'端存在相当大的序列差异，但5'端保守。滑液支原体基因组DNA的Southern印迹分析表明，启动子区域和保守的5'编码序列的一部分以单拷贝形式存在，而编码序列的其余部分以多拷贝形式存在。发现基因组的一个9.7 kb片段包含八个与vlhA部分同源的串联重复区域，所有这些区域都缺乏推定的启动子区域和vlhA的单拷贝5'端，但延伸到3'编码序列的四个不同重叠区域之一。对滑液支原体连续克隆的检查表明，假基因与表达的vlhA之间发生单向重组，假基因序列重复，而表达基因中先前可见的相应区域丢失。vlhA基因两个变体5'端的表达表明，它们与针对该区域的单克隆抗体的反应不同。这些数据表明，滑液支原体中vlhA抗原变异的控制是通过多个基因转换事件实现的，利用一系列编码序列生成嵌合表达基因，在编码血凝素的区域产生变异的潜力最大。因此，已采用完全不同的机制来控制同源基因家族中的抗原变异。
