Haem Biosynthesis, Utilisation, and Acquisition in Ciliates and other Protists

Haem is an important prosthetic group required for a variety of essential biological processes, such as respiration, as a cofactor of cytochromes, and transport of oxygen as a component of globin proteins. The ability to synthesise or obtain haem from the environment is crucial for survival of most organisms, and protists possess diverse mechanisms to achieve this. Through computational analyses of genomes and transcriptomes from a wide range of protists, novel insights into haem requirements were gained. This work revealed new information on haem biosynthetic pathways of the ciliate species of the spirotrich class, demonstrating several of these organisms encode the near complete biosynthetic pathways to synthesise haem de novo. Additionally, presence of the C5 pathway for production of ALA was predicted in five ciliate species, suggesting a new route for production of this haem precursor as previously only the C4 pathway had been identified in ciliates. Where ciliates lack the requisite enzymes to make haem, the presence of haem acquisition systems in these species was analysed. This work provides new explanations for how certain protists obtain and utilise an important cofactor, which has broader implications for niche adaptation and evolution of these poorly-understood organisms.