DUAL FUNCTIONS OF RIBOSOME RECYCLING FACTOR IN PROTEIN-BIOSYNTHESIS -DISASSEMBLING THE TERMINATION COMPLEX AND PREVENTING TRANSLATIONAL ERRORS

We summarize in this communication the data supporting the two functio ns of ribosome recycling factor (RRF, originally called ribosome relea sing factor). The first described role involves the disassembly of the termination complex which consists of mRNA, tRNA and the ribosome bou nd to the mRNA at the termination codon. This process is catalyzed by two factors, elongation factor G (EF-G) and RRF. RRF stimulated protei n synthesis as much as eight-fold in the in vitro lysozyme synthesis s ystem, when ribosomes were limiting. In the absence of RRF, ribosomes remain mRNA-bound at the termination codon and translate downstream co dons. In the in vitro system, the site of reinitiation is the triplet codon 3' to the termination codon. RRF is an essential protein for bac terial life. Temperature sensitive (ts) RRF mutants were isolated and in vivo translational reinitiation due to inactivation of ts RRF was d emonstrated using the beta-galactosidase reporter gene placed downstre am from the termination codon. A second function of RRF involves preve nting errors in translation. In polyphenylalanine synthesis programmed by polyuridylic acid, misincorporation of isoleucine, leucine or a mi xture of amino acids was stimulated upto 17-fold when RRF was omitted from the in vitro system. RRF did not influence the large error (10-fo ld increase) induced by streptomycin. This means that RRF participates not only in the disassembly of the termination complex but also in pe ptide elongation. Extending this concept and its conventional role for releasing ribosomes from mRNA, involvement of RRF in the reinitiation in the 3A' system (a construct using S aureus protein A, a collaborat ive work with Dr Isaksson), in programmed frame shifting, in trans-tra nslation with 10Sa RNA (collaborative work with Dr Muto), and in the r einitiation downstream from the ORF A of the IS 3 (insertion sequence of a transposon, collaborative work with Dr Sekine) are discussed on t he basis of preliminary data to be published elsewhere. Finally, we re view the known RRF sequences from various organisms including eukaryot es and discuss the possible mechanism for disassembly of the eukaryoti c termination complex.