Question

The fungus Fusarium fujikuroi (F. fujikuroi) is a plant pathogen that can infect and destroy corn crops. Some corn varieties are less susceptible to F. fujikuroi than others. A recent study showed that less-susceptible varieties produced and exported an miRNA that targets the fungal gene RROT. The miRNA produced and exported from the corn and then taken up by F. fujikuroi. This miRNA is completely complementary to a highly conserved region of the RROT coding sequence. In fungi such as F. fujikuroi, the RROT gene codes for a transcriptional co-activator. Like many co-activators, RROT is a protein that assists transcription factors in binding to DNA. Even though the miRNA is highly specific to the mRNA of the RROT gene, researchers found that when F. fujikuroi was exposed to this miRNA, expression of a number of other genes in the fungus were down-regulated. Explain how expression of genes other than RROT could be down-regulated.

Answer 1

The down-regulation of genes other than RROT in Fusarium fujikuroi when exposed to miRNA can occur due to the disruption of gene regulatory networks, where the absence of the co-activator RROT hinders the activation of multiple genes, or from off-target effects where the miRNA partially pairs with other mRNA sequences.

Step-by-step explanation:

The expression of genes other than RROT could be down-regulated in Fusarium fujikuroi when exposed to a specific miRNA because of the interconnected nature of the fungal gene regulatory networks. RROT, acting as a transcriptional co-activator, likely assists in the activation of multiple genes. When the miRNA completely complementary to RROT is taken up by the fungus, it can pair with the mRNA of RROT, leading to the degradation or blockage of this mRNA, and thus, preventing the translation of the RROT protein. Without this co-activator, the transcription factors which RROT assists could fail to properly bind to DNA, leading to a cascade effect where several genes that rely on the transcriptional co-activation provided by RROT become down-regulated.

Additionally, if RROT is responsible for the activation of a suite of genes necessary for stress response or nutrient acquisition, the down-regulation of RROT can indirectly lead to the down-regulation of these additional genes. Finally, it is also possible that a single miRNA molecule is acting not only on RROT but may also have off-target effects. This miRNA might incompletely pair with other mRNAs leading to repression of genes that have sequences partially complementary to the miRNA, further influencing the down-regulation of multiple genes in F. fujikuroi.