The assembly of ribosomes in HeLa cell nucleoli.

The order in which ribosomal proteins become associated with nucleolar pre-rRNA was studied by measuring the kinetics of label accumulation in ribosomal proteins isolated from the cytoplasmic ribosomes, as well as by two-dimensional polyacrylamide gel electrophoresis of protein extracted from ribosome precursor particles. Tentative identifications of early-adding and late-adding groups of proteins have been made.