Ahn J H, Walton J D
Department of Energy-Plant Research Laboratory, Michigan State University, East Lansing 48824, USA.
Plant Cell. 1996 May;8(5):887-97. doi: 10.1105/tpc.8.5.887.
Race 1 isolates of the filamentous fungus Cochliobolus carbonum are exceptionally virulent on certain genotypes of maize due to production of a cyclic tetrapeptide, HC-toxin. In crosses between toxin-producing (Tox2+) and toxin-nonproducing (Tox2-) isolates, toxin production segregates in a simple 1:1 pattern, suggesting the involvement of a single genetic locus, which has been named TOX2. Earlier work had shown that in isolate SB111, TOX2 consists in part of two copies of a gene, HTS1, that encodes a 570-kD cyclic peptide synthetase and is lacking in Tox2- isolates. The genomic structure of TOX2 and the relationship between the two copies of HTS1 have now been clarified by using pulsedfield gel electrophoresis and physical mapping. In isolate SB111, both copies of HTS1 are on the largest chromosome (3.5 Mb), which is not present in the related Tox2- strain SB114. Two other genes known or thought to be important for HC-toxin biosynthesis, TOXA and TOXC, are also on the same chromosome in multiple copies. Other independent Tox2+ isolates also have two linked copies of HTS1, but in some isolates the size of the chromosome containing HTS1 is 2.2 Mb. Evidence obtained with Tox2+ -unique and with random probes is consistent with a reciprocal translocation as the cause of the difference in the size of the HTS1-containing chromosome among the Tox2+ isolates studied here. Physical mapping of the 3.5-Mb chromosome of SB111 that contains HTS1 using rare-cutting restriction enzymes and engineered restriction sites was used to map the chromosome location of the two copies of HTS1 and the three copies of TOXC. The results indicate that TOX2 is a complex locus that extends over more than 500 kb. The capacity to produce HC-toxin did not evolve by any single, simple mechanism.
丝状真菌玉米炭疽菌（Cochliobolus carbonum）的第1族分离株对某些玉米基因型具有极强的毒性，这是因为它们能产生一种环四肽，即HC毒素。在产生毒素的（Tox2 +）和不产生毒素的（Tox2 -）分离株之间的杂交中，毒素产生以简单的1:1模式分离，这表明涉及一个单一的基因座，该基因座被命名为TOX2。早期的研究表明，在分离株SB111中，TOX2部分由基因HTS1的两个拷贝组成，该基因编码一种570-kD的环肽合成酶，而在Tox2 -分离株中则不存在。现在，通过使用脉冲场凝胶电泳和物理图谱分析，已经阐明了TOX2的基因组结构以及HTS1两个拷贝之间的关系。在分离株SB111中，HTS1的两个拷贝都位于最大的染色体（3.5 Mb）上，而相关的Tox2 -菌株SB114中不存在该染色体。另外两个已知或被认为对HC毒素生物合成很重要的基因，TOXA和TOXC，也以多个拷贝存在于同一条染色体上。其他独立的Tox2 +分离株也有两个连锁的HTS1拷贝，但在一些分离株中，含有HTS1的染色体大小为2.2 Mb。用Tox2 +特有的探针和随机探针获得的证据与相互易位一致，这是此处研究的Tox2 +分离株中含HTS1染色体大小差异的原因。使用稀有切割限制酶和工程限制位点对SB111中包含HTS1的3.5-Mb染色体进行物理图谱分析，以确定HTS1的两个拷贝和TOXC的三个拷贝在染色体上的位置。结果表明，TOX2是一个复杂的基因座，延伸超过500 kb。产生HC毒素的能力并非通过任何单一、简单的机制进化而来。
