Molecular analysis and evolution of gibberellin biosynthetic gene clusters in fungi
BETTINA TUDZYNSKI
Institut für Botanik
Westfälische Wilhelms Universität Münster
Schlossgarten 3
48149 Münster
tudzynsb@uni-muenster.de
http://www.uni-muenster.de/Biologie.Botanik/agtudzynski/Tudzynsk.htm
Gibberellins (GAs) are diterpenoid plant hormones that are also produced as secondary metabolites by the ascomycete Fusarium fujikuroi (teleomorph: Gibberella fujikuroi) and some other fungi and bacteria. In the last years, the GA biosynthetic genes of higher plants and F. fujikuroi (gene cluster) have been cloned and functionally characterized. Due to fundamental differences in the GA-biosyntheses at the chemical (pathway), biochemical (enzymes) and genetic levels, the hypothesis of a horizontal gene transfer from the plants to the fungus can now be excluded. To elucidate the origin of GA- or similar gene clusters in fungi, we investigated the GA gene cluster in a second, not closely related GA-producing fungus, Sphaceloma manihoticola. This fungus is a cassava pathogen causing superelongation of the internodes similar to F. fujikuroi on rice. In contrast to F. fujikuroi, the fungus produces GA4 and GA9 as final products, which are intermediates of the GA pathway in F. fujikuroi. No traces of GA1, GA7 and GA3 were found suggesting that the 13-hydroxylase (P450-3) and the GA4 desaturase (des), catalyzing the last two steps in F. fujikuroi, are missing or at least inactive in S. manihoticola.
The screening of a genomic library and subsequent chromosome walking revealed a GA biosynthesis cluster similar to that in F. fujikuroi. As in Fusarium, the two monooxygenase genes (P450-1 and P450-4) as well as the ggs2 and cps/ks homologous genes share bidirectional promoters. The order and direction of transcription of the putative GA genes differ in some cases from that in the Fusarium cluster. As already expected, homologues of P450-3 and des, localized at the right and left borders in the Fusarium gene cluster are missing in S. manihoticola.
A BLAST search for cps/ks-and ggs-like genes, which are physically linked sharing the same promoter, displayed homologous complexes in the published genomes of Magnaporthe grisea and Phoma betae. These findings lead to the suggestion of a fungi-specific evolution of diterpenoid gene clusters.
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