Human mitochondrial ribosomes can switch structural tRNAs - but when and why?

High resolution cryoEM of mammalian mitoribosomes revealed the unexpected presence of mitochondrially encoded tRNA as a structural component of mitochondrial large ribosomal subunit (mtLSU). Our previously published data identified that only mitochondrial (mt-) tRNA(Phe) and mt- tRNA(Val) can be incorporated into mammalian mt-LSU and within an organism there is no evidence of tissue specific variation. When mt- tRNA(Val) is limiting, human mitoribosomes can integrate mt- tRNAPhe instead to generate a translationally competent monosome. Here we discuss the possible reasons for and consequences of the observed plasticity of the structural mt-tRNA integration. We also indicate potential direction for further research that could help our understanding of the mechanistic and evolutionary aspects of this unprecedented system.