Shim Won-Bo, Dunkle Larry D
Department of Plant Pathology and Microbiology, The Program for the Biology of Filamentous Fungi, Texas A&M University, College Station, Texas 77843-2132, USA.
Mycologia. 2005 Mar-Apr;97(2):349-55. doi: 10.3852/mycologia.97.2.349.
Two fungal pathogens, Cercospora zeae-maydis Groups I and II, cause gray leaf spot of maize. During the sequencing of a cosmid library from C. zeae-maydis Group I, we discovered a sequence with high similarity to Maggy, a transposable element from Magnaporthe grisea. The element from C. zeae-maydis, named Malazy, contained 194-base-pair terminal repeats and sequences with high similarity to reverse transcriptase and integrase, components of the POL gene in the gypsy-like retrotransposons in fungi. Sequences with similarity to other POL gene components, protease and ribonuclease, were not detected in Malazy. A single copy of the element was detected by PCR and Southern analyses in all six North American isolates of C. zeae-maydis Group I but was not detected in the four isolates of C. zeae-maydis Group II from three continents or in phylogenetically related species. Fragments of the core domains of reverse transcriptase and integrase contained a high frequency of stop codons that were conserved in all six isolates of Group I. Additional C:G to T:A transitions in occasional isolates usually were silent mutations, while two resulted in isolate-specific stop codons. The absence of Malazy from related species suggests that it was acquired after the divergence of C. zeae-maydis Groups I and II. The high frequency of stop codons and the presence of a single copy of the element suggest that it was inactivated soon after it was acquired. Because the element is inactive and because reading frames for other genes were not found in sequences flanking the element, Malazy does not appear to be the cause of differences leading to speciation or genetic diversity between C. zeae-maydis Groups I and II.
两种真菌病原体，玉米尾孢菌I组和II组，可引发玉米灰斑病。在对玉米尾孢菌I组的黏粒文库进行测序时，我们发现了一段与稻瘟病菌的转座元件Maggy高度相似的序列。来自玉米尾孢菌的该元件名为Malazy，含有194个碱基对的末端重复序列，以及与逆转录酶和整合酶高度相似的序列，这两种酶是真菌中类gypsy逆转座子POL基因的组成部分。在Malazy中未检测到与其他POL基因组成部分（蛋白酶和核糖核酸酶）相似的序列。通过PCR和Southern分析在所有6株北美玉米尾孢菌I组分离株中检测到该元件的单拷贝，但在来自三大洲的4株玉米尾孢菌II组分离株或系统发育相关物种中未检测到。逆转录酶和整合酶核心结构域的片段含有高频终止密码子，这些密码子在I组的所有6株分离株中都是保守的。偶尔分离株中额外的C:G到T:A转换通常是沉默突变，而有两个导致了分离株特异性的终止密码子。相关物种中不存在Malazy，这表明它是在玉米尾孢菌I组和II组分化之后获得的。终止密码子的高频出现以及该元件单拷贝的存在表明它在获得后不久就失活了。由于该元件是无活性的，并且在该元件侧翼序列中未发现其他基因的阅读框，因此Malazy似乎不是导致玉米尾孢菌I组和II组之间物种形成或遗传多样性差异的原因。
