Special focus on the ribosome life cycle

Special focus on the ribosome life cycleIn all cells and tissues, protein production is carried out by a ubiquitous machine -the ribosome, a three Mega Dalton macromolecular complex that is essential across all domains of life.It was 65 years ago, in 1958, that the ribosome as protein production machinery was first named by Richard Roberts [1][2][3].Five years later, in January 1963, Jonathan Warner, Paul Knopf and Alex Rich published the first paper on the characterization of polyribosomes, or polysomes, from rabbit reticulocyte lysate [4].The very same year, work by Alfred Gierer in Tubingen, also using reticulocytes [5] and Hans Noll in Pittsburgh, using rat liver [6] independently suggested that protein synthesis was conducted on structures with multiple ribosomes scanning a single mRNA, and in 1964, description of the A(cceptor) and P(eptidyl) sites (the E, or exit, site was only identified by Knut Nierhaus in 1981 [7]) and mRNA and tRNA, followed [8-14].In the span of a little over a year and a half, views of protein synthesis evolved from a vague idea of an interaction between mRNA, tRNA and ribosomes to a basic model that guides ongoing research into mechanisms of translation regulation to this day: ribosomes move from one end of an mRNA to another, and each has a site for a tRNA attached to the growing polypeptide and a site for the incoming tRNA attached to an amino acid.It also became clear very early on that the ribosome itself is composed of both RNA and proteins and investigation into its composition and biogenesis began in the 1950s and 60s, in bacteria and eukaryotes, respectively [3].In 1962, Klaus Scherrer, Harriet Latham, and James E. Darnell demonstrated the existence of precursor ribosomal RNAs (rRNAs) in HeLa cells [15], while R. J. Britten, B. J. McCarthy, and Richard Roberts suggested that ribosomes are formed in a stepwise manner [16,17].The nucleolus was identified as the site of ribosome biogenesis only 2 years later [18], the same year as rRNA base methylation [19] and both rRNA and 'nascent' ribosomes of different molecular mass were isolated from human nuclei for the first time in 1967 [15,19,20].It was in 1972 that a study by Kumar & Warner on nuclear vs cytoplasmic ribosomes in human cells revealed a considerable difference in protein content [21].At the time, these 'extra proteins' within nuclear ribosomes werein retrospect, correctly -hypothesized to function in the processing of rRNA precursors, and while ribosomes were moved to the cytoplasm, these 'extra proteins' stayed behind and were surmised to be re-utilized in further rounds of ribosome production [5].These 'extra' proteins are now known to be the many ribosome biogenesis factors that are required to modify, process, and fold the ribosomal RNAs (rRNA) and assemble them with the ribosomal proteins (r-proteins) into mature ribosomes prior to their

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