A cell fractionation procedure is described which allowed, by use of MOPC 21 (P3K) mouse plasmocytoma cells in culture, the separation of the cytoplasmic free and membrane-bound ribosomes in fractions devoid of mutual cross-contamination, and in which the polyribosomal structure was entirely preserved. This was achieved by sedimentation on a discontinuous sucrose density gradient in which the t...

Ribosomes are the molecular machines that translate mRNAs into proteins. The synthesis of ribosomes is therefore a fundamental cellular process and consists in the ordered assembly of 79 ribosomal proteins (r-proteins) and four ribosomal RNAs (rRNAs) into a small 40S and a large 60S ribosomal subunit that form the translating 80S ribosomes. Most of our knowledge concerning this dynamic multi-st...

J-proteins and Hsp70 chaperones function together in diverse cellular processes. We identified a cytosolic J-protein, Jjj1, of Saccharomyces cerevisiae that is associated with 60S ribosomal particles. Unlike Zuo1, a 60S subunit-associated J-protein that is a component of the chaperone machinery that binds nascent polypeptide chains upon their exit from the ribosome, Jjj1 plays a role in ribosom...

Ribosomal RNAs (rRNAs) act as scaffolds and ribozymes in ribosomes, and these functions are modulated by post-transcriptional modifications. However, the biological role of base methylation, a well-conserved modification of rRNA, is poorly understood. Here, we demonstrate that a nucleolar factor, nucleomethylin (NML; also known as RRP8), is required for the N(1)-methyladenosine (m(1)A) modifica...

The plant viral re-initiation factor transactivator viroplasmin (TAV) activates translation of polycistronic mRNA by a re-initiation mechanism involving translation initiation factor 3 (eIF3) and the 60S ribosomal subunit (60S). QJ;Here, we report a new plant factor-re-initiation supporting protein (RISP)-that enhances TAV function in re-initiation. RISP interacts physically with TAV in vitro a...

Two strains of Saccharomyces cerevisiae were constructed that are conditional for synthesis of the 60S ribosomal subunit protein, L16, or the 40S ribosomal subunit protein, rp59. These strains were used to determine the effects of depriving cells of either of these ribosomal proteins on ribosome assembly and on the synthesis and stability of other ribosomal proteins and ribosomal RNAs. Terminat...

Ribosome biogenesis in eukaryotes involves the transcription, folding, and processing of ribosomal RNA (rRNA), as well as the concomitant assembly of ribosomal proteins. Several hundred trans-acting assembly factors also play a role in the complex process of ribosome biogenesis. Investigations of the construction of ribosomes have focused primarily on the roles of these assembly factors. Little...

BACKGROUND NIR was identified as an inhibitor of histone acetyltransferase and it represses transcriptional activation of p53. NIR is predominantly localized in the nucleolus and known as Noc2p, which is involved in the maturation of the 60S ribosomal subunit. However, how NIR functions in the nucleolus remains undetermined. In the nucleolus, a 47S ribosomal RNA precursor (pre-rRNA) is transcri...

The assembly of ribosomal subunits in eukaryotes is a complex, multistep process so far mostly studied in yeast. In S. cerevisiae, more than 200 factors including ribosomal proteins and trans-acting factors are required for the ordered assembly of 40S and 60S ribosomal subunits. To date, only few human homologs of these yeast ribosome synthesis factors have been characterized. Here, we used a s...