Termination and post-termination events in eukaryotic translation

Translation termination in eukaryotes occurs in response to a stop codon in the ribosomal A-site and requires two release factors (RFs), eRF1 and eRF3, which bind to the A-site as an eRF1/eRF3/GTP complex with eRF1 responsible for codon recognition. After GTP hydrolysis by eRF3, eRF1 triggers hydrolysis of the polypeptidyl-tRNA, releasing the completed protein product. This leaves an 80S ribosome still bound to the mRNA, with deacylated tRNA in its P-site and at least eRF1 in its A-site, which needs to be disassembled and released from the mRNA to allow further rounds of translation. The first step in recycling is dissociation of the 60S ribosomal subunit, leaving a 40S/deacylated tRNA complex bound to the mRNA. This is mediated by ABCE1, which is a somewhat unusual member of the ATP-binding cassette family of proteins with no membrane-spanning domain but two essential iron-sulfur clusters. Two distinct pathways have been identified for subsequent ejection of the deacylated tRNA followed by dissociation of the 40S subunit from the mRNA, one executed by a subset of the canonical initiation factors (which therefore starts the process of preparing the 40S subunit for the next round of translation) and the other by Ligatin or homologous proteins. However, although this is the normal sequence of events, there are exceptions where the termination reaction is followed by reinitiation on the same mRNA (usually) at a site downstream of the stop codon. The overwhelming majority of such reinitiation events occur when the 5'-proximal open reading frame (ORF) is short and can result in significant regulation of translation of the protein-coding ORF, but there are also rare examples, mainly bicistronic viral RNAs, of reinitiation after a long ORF. Here, we review our current understanding of the mechanisms of termination, ribosome recycling, and reinitiation after translation of short and long ORFs.