Unexpected Production of Cysteine Amino Acid Found in Coral

Model organisms in scientific research

From their use in basic biology to the development and testing of novel therapeutics, model organisms have helped to solve key scientific questions in experiments that are either impractical or unethical to conduct in humans. Important examples include rats, mice, zebrafish and non-human primates, often chosen for their likeness to humans in terms of anatomy, physiology or immunological response.

However, not all results gathered using model organisms can be translated to humans – an acknowledged limitation of their use. A new genetic study of corals of the genus Acropora has emphasized this point further still, uncovering an unexpected pathway for the biosynthesis of an essential amino acid.

An alternate pathway for the production of a key amino acid

Cysteine (Cys) is an amino acid found in high abundance across many biological processes, such as protein synthesis and metabolism. In animals, the synthesis of cysteine was thought to occur via one specific pathway, known as the transsulfuration pathway, which involves the enzyme cystathionine β-synthase (CBS) encoded by the CBS gene.

Researchers at the King Abdullah University of Science and Technology (KAUST)  were studying corals of the Acroporoa genus, with the aim of generating a high-quality genome of Acropora loripes, “a valuable genomic resource” for future research. “We weren’t searching for possible cysteine biosynthesis in Acropora,” says Dr. Octavio Salazar, a postdoc in The Coral Symbiomics lab at KAUST, and lead author of the study.

And yet – that’s exactly what the researchers found; a surprise, considering that previous work had suggested the CBS gene had been lost in these corals, meaning they must rely on symbiotic relationships with algae to receive cysteine.

Once the genome was complete, Salazar and colleagues searched for proof that the CBS gene was in fact absent. It was, but Salazar remained skeptical.

“I started searching the genome for genes encoding for enzymes that looked similar to those in other known cysteine biosynthesis pathways, such as those found in fungi and bacteria,” says Salazar. “I was quite surprised to find two enzymes in the coral with similarities to a recently identified alternative cysteine biosynthesis pathway in fungi.”

The research team used yeast mutants that are completely unable to synthesize cysteine, and inserted the genes found in Acropora. Interestingly, the mutant yeast began to produce cysteine, indicating that the enzymes encoded by the genes found in the coral could synthesize the amino acid in vivo.

Implications for research using animal models

When looking further afield in the genomic landscape, Salazar and colleagues found that the genes were also present in the genomes of all animal phyla, except for vertebrates, nematodes and arthropods. As these three groups are the source of the most common model organisms used in scientific research, the team advise caution when it comes to overlying on findings from animal models.

“This study proves the value of keeping an open mind when it comes to studying living creatures,” says principal investigator Professor Manuel Aranda from KAUST. “Sometimes knowledge can put you in a box; if you analyze data using only what you think you know, you may well miss something. Our Acropora genome will be hugely valuable for future studies and who knows, it could reveal other unexpected details along the way.”

Reference: Salazar OR, N. Arun P, Cui G, et al. The coral Acropora loripes genome reveals an alternative pathway for cysteine biosynthesis in animals. Sci Adv. 2022 8(38):eabq0304. doi:10.1126/sciadv.abq0304.

This article is a rework of a press release issued by KAUST university. Material has been edited for length and content.