Mitochondria and Respiration

This chapter covers the functions of mitochondria in filamentous fungi. Mitochondrial respiration occurs via transfer of electrons from reduced electron carriers to molecular oxygen. This is accomplished using an electron transport chain housed in the mitochondrial inner membrane (MIM). Fungal mitochondrial genomes are AT-rich DNAs that map as circular molecules. However, several studies suggest that fungal mtDNAs are actually long, concatenated linear molecules. The study of fungal mitochondrial introns has been an active area of research that has yielded a number of significant findings. There are numerous reports of RNA elements being associated with mitochondria. Though they are formally classified as mitoviruses, they can also be considered plasmids based on their structural similarities to linear mt plasmids and lack of an extracellular form. The large number of different proteins that are found in mitochondria (probably about 1,000 for fungi) and the limited coding capacity of mtDNA emphasize the fact that most mitochondrial proteins are encoded by nuclear genes, translated in the cytosol, and imported into the organelle. The chapter attempts to define the basic import process and summarizes the literature while pointing out contributions that have involved filamentous fungi-almost entirely N. crassa. Filamentous fungi are generally considered immortal. Podospora anserina is an exception to this rule, as it has a defined vegetative- growth life span.