Fig. 5.

Distributions of AFs—models. a) Isoelectric balance. The nascent rRNA recruits basic proteins to its successive domains. Such proteins then attract less basic AFs. These AFs, in turn, preferentially bind basic ribosomal proteins, while other ribosomal proteins bind directly to rRNA. The layered model incorporates these observations, suggesting that they reflect a widespread tendency. There is however no reason to expect that intermediates are organized as concentric spheres. b) Sequential nucleation, recruitment and consolidation of AFs from a shared reservoir. From left-to-right: initiator RNA-associated proteins (blue, a, b, c, etc.) bind successive sites along transcripts, latent AFs (multicolored) are recruited to the initiators from the shared reservoir throughout the nucleolus, the resulting rRNPs then consolidate into composite units resembling those of Fig. 1e. This sequence of events is depicted in association with the inner layer, as is appropriate for pre-40S intermediates. In this diagram, all AFs that bind a given initiator have the same shape and color. This is not intended to imply that they are chemically identical. In the reservoir, some AFs are monomers, while others (possibly the majority) are associated with other AFs.