Small molecular weight RNA species (smwRNAs) were studied in rat liver nuclei with and without chromatin as well as with and without nuclear envelope and nucleoplasm. From all the species identified, only two, N5 and 5Sb, were related to ribosomes. The others were localized exclusively in the nuclear skeleton or the spongelike network that was described in the preceding communication. This network or protein matrix contains a less abundant but exclusive set of molecules designated 5Sa, N1, and 4.5S, as well as other more abundant molecules which also exist in rat liver endoplasmic reticulum but not in polysomes or postribosomal RNP complexes. The smwRNAs behave like HnRNA; they remain located in the nuclear skeleton when nuclei are deprived of nucleoplasm and chromatin. With the information presently available, it is not possible to know whetherer both species are in the same or different RNP complexes and whether some of the smwRNAs contribute to the architecture of the nuclear skeleton. Distinct from any other nuclear RNA species, smwRNAs have two unique properties: facility of extraction, and resistance to nuclear ribonuclease digestion.

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