g., refs. [39–41]. However, this scenario struggles to explain why secondary metabolite genes appear to have a different evolutionary trajectory than genes for primary metabolism, i.e., to what extent there are positively selected genetic mechanisms that promote diversity in secondary metabolite capacity at the expense of stability, such as transposable elements, PDGFR inhibitor sub-telomeric instability, and chromosomal translocations [10, 22]. Taxonomic distribution of TOXE Since the discovery of this atypical transcription factor in 1998 [26], TOXE has
been found in only a handful of other organisms, all fungi. Besides C. carbonum and A. jesenskae, reasonably strong orthologs of TOXE are present only in Pyrenophora tritici-repentis, P. teres, Colletotrichum gloeosporioides, Setosophaeria turcica, Fusarium incarnatum (APS2), and Glomerella cingulata (based on GenBank and JGI as of March, 2013). The first four fungi are in the Dothideomycetes
and the second two are in the Sordariomycetes. Genes with reasonable amino acid identity and structure (i.e., containing both a bZIP DNA binding domain and ankyrin repeats) are not present in any NSC 683864 cell line other fungus including other species of Cochliobolus and Fusarium. TOXE showed the lowest percent amino acid identity between C. carbonum and A. jesenskae (58-64%) of any of the TOX2 proteins, and the next best ortholog (APS2 of F. incarnatum) shares only 32% amino acid identity. That these are all true orthologs can be deduced by the strong conservation of the bZIP DNA binding
region at the N terminus, the ankyrin repeats at the C terminus, and by the fact that APS2 has an experimentally determined role in regulating the biosynthesis of a secondary metabolite chemically similar to HC-toxin [14]. Apparently, the Fludarabine mouse specific amino acid sequence of most of the TOXE protein is not essential for its activity. This is reminiscent of the transcription factor aflR in Aspergillus flavus and A. nidulans; the two proteins are functional orthologs despite only 33% amino acid identity [42]. APS2 is required for expression of the apicidin biosynthetic genes [14], but the functions of the other TOXE orthologs are not known. In P. tritici-repentis, G. cingulata, and S. turcica, the TOXE orthologs (JGI identifiers Pyrtr1|12016, Gloci1|1721714, BCKDHA and Settu1|170199, respectively) are immediately adjacent to four-module NRPS genes, suggesting that the TOXE orthologs in these fungi have a role in regulating secondary metabolite production like they do in C. carbonum and F. incarnatum[21, 22, 43]. Are there orthologs of the TOX2 genes in other fungi? Recently, two other fungi in the Pleosporaceae, P. tritici-repentis and S. turcica, were reported to have the HTS1 gene [21]. This conclusion was based on the presence of a four-module NRPS clustered with genes similar to TOXD, TOXA, and TOXE. Putative orthologs of TOXC, TOXD, and TOXG were found elsewhere in the genomes of these two fungi.
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