Mikalsen Bjørg, Boison Gudrun, Skulberg Olav M, Fastner Jutta, Davies William, Gabrielsen Tove M, Rudi Knut, Jakobsen Kjetill S
Department of Biology, University of Oslo, Norway.
J Bacteriol. 2003 May;185(9):2774-85. doi: 10.1128/JB.185.9.2774-2785.2003.
Toxic Microcystis strains often produce several isoforms of the cyclic hepatotoxin microcystin, and more than 65 isoforms are known. This has been attributed to relaxed substrate specificity of the adenylation domain. Our results show that in addition to this, variability is also caused by genetic variation in the microcystin synthetase genes. Genetic characterization of a region of the adenylation domain in module mcyB1 resulted in identification of two groups of genetic variants in closely related Microcystis strains. Sequence analyses suggested that the genetic variation is due to recombination events between mcyB1 and the corresponding domains in mcyC. Each variant could be correlated to a particular microcystin isoform profile, as identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. Among the Microcystis species studied, we found 11 strains containing different variants of the mcyABC gene cluster and 7 strains lacking the genes. Furthermore, there is no concordance between the phylogenies generated with mcyB1, 16S ribosomal DNA, and DNA fingerprinting. Collectively, these results suggest that recombination between imperfect repeats, gene loss, and horizontal gene transfer can explain the distribution and variation within the mcyABC operon.
有毒微囊藻菌株通常会产生几种环状肝毒素微囊藻毒素的异构体，已知的异构体超过65种。这归因于腺苷化结构域的底物特异性较为宽松。我们的结果表明，除此之外，微囊藻毒素合成酶基因的遗传变异也会导致变异性。对模块mcyB1中腺苷化结构域的一个区域进行遗传特征分析，结果在密切相关的微囊藻菌株中鉴定出两组遗传变异体。序列分析表明，遗传变异是由于mcyB1与mcyC中相应结构域之间的重组事件所致。通过基质辅助激光解吸电离飞行时间质谱法鉴定，每个变异体都可能与特定的微囊藻毒素异构体谱相关。在所研究的微囊藻物种中，我们发现11个菌株含有mcyABC基因簇的不同变异体，7个菌株缺乏这些基因。此外，用mcyB1、16S核糖体DNA和DNA指纹图谱构建的系统发育树之间不存在一致性。总体而言，这些结果表明，不完全重复序列之间的重组、基因丢失和水平基因转移可以解释mcyABC操纵子内的分布和变异。
