tRNA-assisted overproduction of eukaryotic ribosomal proteins

Structural studies of eukaryotic ribosomes are complicated by the tendency of their constituent proteins to be expressed at very low levels in Escheri chia coli. We find that this is mainly due to their exceptionally high cont ent of AGA/AGG arginine codons, which are poorly utilized by the bacterial translational machinery. In fact, we could overcome this limitation by the combined use of a T7 RNA polymerase expression vector and a plasmid carryin g the E. coli gene argU, which encodes the minor tRNA(Arg) species that rea ds AGA/AGG codons. In this system, five cytoplasmic ribosomal proteins from three different eukaryotic lineages (Saccharomyces cerevisiae S8, L13, and L14; Arabidopsis thaliana L13; and Homo sapiens L7) could be overexpressed to up to 50% of total bacterial protein and were purified to homogeneity i n tens of milligrams amounts. The purification procedure simply involved me tal affinity chromatography followed, in some cases, by an additional hepar in chromatography step. Recombinant polypeptides bound RNA with high affini ty (K-d between 50 and 300 nM). This novel overexpression/purification stra tegy will allow the production of high amounts of most eukaryotic ribosomal proteins in a form suitable for structural and functional studies. Coupled with recently completed and ongoing whole-genome sequencing projects, it w ill facilitate the molecular characterization of the eukaryotic ribosome. ( C) 2000 Academic Press.