The nucleolus was identified early on as a site that made a lot of RNA (Caspersson and Schultz, 1940); later that RNA was shown to have metabolic dynamics distinct from those of chromosomally-derived RNA (McMaster-Kaye and Taylor, 1958). But the function of RNA made in the nucleolus was obscure. Two papers from Jan-Erik Edström (Edström, 1960; Edström et al., 1961) gave the first clues.

Edström's approach was simply to look at the base compositions of different populations of RNA. He was not the first to do so. Vincent (1952) had found that base compositions did not match up for nucleolar and cytoplasmic RNAs. But he had used bulk isolation of full-grown starfish oocytes to gather material. Edström opted instead to use young, growing oocytes isolated via microdissection. His first subject was oocytes from spiders—“a good choice of materials,” he says, “because the building was old and there were plenty of them along the walls”—with the later study using starfish oocytes.

Both studies came to essentially the same conclusion: nucleolar and cytoplasmic RNA had similar base compositions, whereas nucleoplasmic RNA was distinct (high A/U and low G/C, like DNA). Thus nucleolar RNA might be the precursor of cytoplasmic RNA, as nuclear RNA was known to move to the cytoplasm.

graphic

RNA from nucleoli (dark stain in center of oocytes) has a similar base composition to cytoplasmic RNA.

EDSTRÖM

This was all that Edström could say in 1960, but by 1961 he was emboldened and enlightened by the discovery of mRNA (Brenner et al., 1961; Gros et al., 1961). The later abstract (Edström et al., 1961) stated that “our data favor a nucleolar origin for the stable part of the ribosomal RNA and a nucleoplasmic one for the unstable part (the messenger RNA).” This idea was explained further in the discussion: “The base analyses indicate…that in case there is a high relative contribution of nucleoplasmic RNA [to the cytoplasmic pool] it is necessary that this RNA should have a considerably shorter life than that [from] the nucleolus. The nucleoplasmic RNA could thus very well fit into the role of messenger RNA.” Shortly afterwards, Perry (1962) used inhibitors to prove that nucleolar RNA was, indeed, the obligatory precursor of cytoplasmic ribosomal components.

Brenner, S., et al. 1961. Nature. 190:576.

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