Derivatives of the yeast mitochondrial ribosomal protein MrpS28 replace ribosomal protein S15 as functional components of the Escherichia coli ribosome.

The mitochondrial ribosomal protein MrpS28 is considerably larger than its eubacterial homolog, Escherichia coli ribosomal protein S15 (Eco S15). Relative to a region of homology that spans the entire length of the bacterial protein, mature MrpS28 is extended by 117 and 48 amino acids at its amino and carboxyl termini, respectively. Both the amino-terminal and S15-like domains of MrpS28 are essential for function in yeast mitochondria. Here, we show that these same two domains function in E. coli. The S15-like domain of MrpS28 alone complements a cold-sensitive mutation in E. coli strain KR121 that gives rise to reduced levels of Eco S15. However, complementation by the S15-like domain of MrpS28 is inefficient when compared with Eco S15. Surprisingly, the amino-terminal domain of MrpS28, which is apparently a unique component of the mitochondrial ribosome and is unable by itself to complement the cold-sensitive phenotype, enhances the ability of the S15-like domain to support growth of KR121 cells at nonpermissive temperatures. Together, these data suggest that the amino-terminal domain contributes to the fundamental properties of MrpS28 involved in the assembly and function of both mitochondrial and E. coli ribosomes.