Visualization of the joining of ribosomal subunits reveals the presence of 80S ribosomes in the nucleus.

In eukaryotes the 40S and 60S ribosomal subunits are assembled in the nucleolus, but there appear to be mechanisms preventing mRNA binding, 80S formation, and initiation of translation in the nucleus. To visualize association between ribosomal subunits, we tagged pairs of Drosophila ribosomal proteins (RPs) located in different subunits with mutually complementing halves of fluorescent proteins. Pairs of tagged RPs expected to interact, or be adjacent in the 80S structure, showed strong fluorescence, while pairs that were not in close proximity did not.

Fluorescent protein tagging confirms the presence of ribosomal proteins at Drosophila polytene chromosomes.

Most ribosomal proteins (RPs) are stoichiometrically incorporated into ribosomal subunits and play essential roles in ribosome biogenesis and function. However, a number of RPs appear to have non-ribosomal functions, which involve direct association with pre-mRNA and transcription factors at transcription sites. The consensus is that the RPs found at these sites are off ribosomal subunits, but observation that different RPs are usually found together suggests that ribosomal or ribosomal-like subunits might be present.