Figure 3.

Induction of non-coding RNA (ncRNA) from the ribosomal cassette. (A) Schematic of a single human rDNA repeat unit, which is composed of a ∼13-kb pre-rRNA coding region flanked by a ∼30-kb intergenic spacer (rIGS). The rIGS transcribes several functional non-coding RNA. Stimuli-specific loci of rIGS produce ribosomal intergenic spacer RNA (rIGSRNA) required for A-body formation. rIGS₂₈RNA and rIGS₁₆RNA/rIGS₂₂RNA are produced under acidotic (yellow) and heat shock (red) conditions, respectively. No function has been ascribed to rIGS₁₈RNA yet. Other ncRNA found in the rIGS include a >10-kb transcript called PNCTR (pyrimidine-rich non-coding transcript) involved in PTBP1 binding (purple), pRNA (green), and antisense PAPAS (brown) involved in rRNA regulation, as well as Alu element-derived (gray boxes) RNA involved in nucleolar architecture. (B) rIGSRNA contain low-complexity sequences comprising of long dinucleotide repeats, as determined by RNA sequencing, RT cloning, and RNA-FISH. This is in contrast to other ncRNA that display high complexity, i.e., possess secondary structure, such as pRNA.