Emb15 encodes a plastid ribosomal assembly factor essential for embryogenesis in maize

Ribosome assembly factors guide the complex process by which ribosomal proteins and the ribosomal RNAs form a functional ribosome. However, the assembly of plant plastid ribosomes is poorly understood. In the present study, we discovered a maize (Zea mays) plastid ribosome assembly factor based on our characterization of the embryo defective 15 (emb15) mutant. Loss of function of Emb15 retards embryo development at an early stage, but does not substantially affect the endosperm, and causes an albino phenotype in other genetic backgrounds. EMB15 localizes to plastids and possesses a ribosome maturation factor M (RimM) domain in the N-terminus and a predicted UDP-GlcNAc pyrophosphorylase domain in the C-terminus. The EMB15 RimM domain originated in bacteria and the UDP-GlcNAc pyrophosphorylase domain originated in fungi; these two domains came together in the ancestor of land plants during evolution. The N-terminus of EMB15 complemented the growth defect of an Escherichia coli strain with a RimM deletion and rescued the albino phenotype of emb15 homozygous mutants. The RimM domain mediates the interaction between EMB15 and the plastid ribosomal protein PRPS19. Plastid 16S rRNA maturation is also significantly impaired in emb15. These observations suggest that EMB15 functions in maize seed development as a plastid ribosome assembly factor, and the C-terminal domain is not important under normal conditions.