Chloramphenicol is a broad-spectrum antibiotic that is active against most bacteria. However, it has varied side effects—such as human aplastic anemia, bone marrow suppression, and gray baby syndrome—that limit its prescription. In a study published today in Scientific Reports, researchers demonstrate the molecular and cellular basis of these adverse effects.
The researchers used the rice blast fungus Magnaportheoryzae as a model organism for the study. “The genome of this fungus has considerably less junk DNA—the noncoding part of the genome that does not code for any functional proteins,” says senior author Takashi Kamakura of Tokyo University of Science. “This, along with the low number of targetable functional proteins, makes the rice blast fungus an ideal model for the screening of novel molecular targets of drugs.”
The rice blast fungus infects plants using a 3-cell structure called the conidium, which has a germ tube with a structure called the appressorium at the end. The appressorium initiates contact with the plant cell and is crucial in the infection cycle. This entire process is known to be dependent on the mitotic division of the single-cell germ tube and cell differentiation. Given that cell differentiation depends on several molecular components, these molecules can also be drug targets.
The researchers first exposed a suspension of the fungal conidia to different concentrations of chloramphenicol and found that, while germination of the conidia or the length of the germ tube remained unaffected, the percentage of appressorium formation significantly dwindled. Next, they screened for chloramphenicol targets in the entire genome of the rice blast fungus. One of the targeted peptides was extremely similar to a highly conserved eukaryotic protein called Dullard that is known to play a role in cell differentiation in eukaryotes.
The researchers found that this protein, which they named MoDullard, was expressed the most at the appressorium formation stage. When they knocked out the MoDullard gene, they found that mutants with no MoDullard were not able to produce the appressorium but were resistant to chloramphenicol, confirming that chloramphenicol targets this protein.
Finally, to determine the target in humans, the team isolated 5 candidate genes from the human genome and transferred them individually into the MoDullard knockouts. One of the peptides, CTDSP1, complemented the lost function of MoDullard in the mutant cells, restoring the ability to produce appressorium.