4 Avermectinポリケチド骨格の生合成に関する分子遺伝学的解析(口頭発表の部)

抄録

The biosynthetic pathway of avermectin has all but been elucidated. The proposed pathway is based on evidence obtained from several studies, including incorporation of labeled precursors into the avermectins and identification of key intermediates produced by biosynthetically blocked mutants or wild type-producing strains. The results of the incorporation of ^<13>C-labeled precursors and analysis of the resulting ^<13>C-labeled avermectin indicate that the avermectin aglycons are derived by head-to-tail condensation of various acyl groups which is similar to the biosynthesis of the other polyketides. The polyketide synthases use the appropriate CoA ester as a primer and add acetate units from malonyl-CoA and propionate units from methylmalonyl-CoA to assemble the polyketides. Avermectin aglycon is formed by addition of the starter unit (2-methylbutyrate or isobutyrate), of 12 acyl condensations in the order P-A-A-A-A-P-P-A-P-A-P-A (P, propionyl; A, acetyl). Within the 95 kbp gene cluster for avermectin biosynthesis, the central 65 kbp segment was found to be required for aglycon biosynthesis by phenotypic analysis of strains containing deletion or insertion mutations in this region. A sequence analysis of the central 65 kbp segment within the gene cluster for avermectin biosynthesis indicated that this segment encodes avermectin polyketide synthases. The avermectin polyketide synthase genes are organized into two converging blocks of ORFs. From the results of sequencing analysis, a feature of the two regions, aveAI and aveAII, is that they encode four kinds of large multifunctional polypeptides contains domains possessing putative fatty acid synthase-like activities. The PKS genes for avermectin biosynthesis are organized into two convergently transcribed clusters of six modules. The avermectin PKS proteins appear to contain two, three, or four synthase units (SU). PKS-1 with two SUs and PKS-2 with four SUs contain modules 1 and 2, and 3-6, respectively, whereas PKS-3 and -4 with three SUs contain modules 7-9, and modules 10-12, respectively. The 12 SUs correspond to the 12 cycles required for synthesis of the initial aglycon.