Eleven of the 55 secondary metabolite clusters were upregulated at the lower temperature, including aflatoxin biosynthesis genes, which were among the most highly upexpressed genes. On average, transcript abundance for the 30 aflatoxin biosynthesis genes was 3300 times greater at 30 °C as compared with 37 °C. The results are consistent with the
view that high temperature negatively affects find more aflatoxin production by turning down transcription of the two key transcriptional regulators, aflR and aflS. Subtle changes in the expression levels of aflS to aflR appear to control transcription activation of the aflatoxin cluster. Aspergillus flavus produces aflatoxins B1 and B2 and causes aflatoxin contamination of preharvest crops such as corn, cotton, peanuts and tree nuts, and postharvest grains during storage (Bhatnagar et al., 1987; Bennett & Klich, 2003). The discovery of the first stable aflatoxin precursor, norsolorinic acid (Bennett, 1981), paved the way
for the elucidation of the aflatoxin biosynthetic pathway, including its intermediates and biosynthetic gene clusters in A. flavus, Aspergillus parasiticus, Aspergillus nidulans (sterigmatocystin as end product), Aspergillus sojae and Aspergillus oryzae (nonfunctional gene cluster) (Brown et al., 1996; Yu et al., 2004a, b). Aflatoxin biosynthesis is affected by many biotic and abiotic factors (Payne & Brown, 1998; Yu et al., 2010). The influence of temperature Selleck 5-Fluoracil on aflatoxin formation has been reported previously (Schroeder & Hein, 1968; Ogundero, 1987). The optimum check details temperature for biosynthesis of aflatoxin and other secondary metabolites is at 30 °C; while the optimum temperature for fungal growth is at about 37 °C but it is less optimal for mycotoxin production. Sequencing of the A. flavus genome facilitated the construction of microarrays, which have been used to study transcriptional
regulation of aflatoxin biosynthesis at different temperatures (OBrian et al., 2007; Georgianna et al., 2008, 2010; Payne et al., 2008; Schmidt-Heydt et al., 2009). These studies identified a large number of genes expressed at high level under low temperature. The effect of temperature on natural antisense transcript expression was also reported (Smith et al., 2008). While microarrays are a robust tool for genome-wide gene expression analysis, they have been plagued by high background and low sensitivity problems. For regulatory genes with low level of expression, microarrays often fail to provide meaningful information about their expression levels. Thus, no published microarray experiments have provided an accurate estimate of the aflR and aflS expression levels. RNA-Seq technology has been successful for transcriptome profiling in a closely related species, A. oryzae (Wang et al., 2010).