Rapid evolution in sequence and length of the nuclear-located gene for mitochondrial L2 ribosomal protein in cereals

The L2 ribosomal protein is typically one of the most conserved proteins in the ribosome and is universally present in bacterial, archaeal, and eukaryotic cytosolic and organellar ribosomes. It is usually 260-270 amino acids long and its binding to the large-subunit ribosomal RNA near the peptidyl transferase center is mediated by a beta-barrel RNA-binding domain with 10 beta strands. In the diverse land plants Marchantia polymorpha (liverwort) and Oryza sativa (rice), the mitochondrial-encoded L2 ribosomal protein is about 500 amino acids long owing to a centrally located expansion containing the beta3-beta4 strand region. We have determined that, in wheat, the functional rpl2 gene has been transferred to the nucleus and much of the plant-specific internal insert has been deleted. Its mRNA is only 1.2 kb, and two expressed copies in wheat encode proteins of 318 and 319 amino acids, so they are considerably shorter than the maize nuclear-located rpl2 gene of 448 codons. Comparative sequence analysis of cereal mitochondrial L2 ribosomal proteins indicates that the mid region has undergone unexpectedly rapid evolution during the last 60 million years.