Description of putative ribosomal RNAs with low abundance, developmental regulation, and the identifier sequence

Three RNA species (5, 2, 0.15 kb) characterized by the repetitive identifier (ID) sequence, expressed constitutively, and at low abundance have been identified in rat L6 muscle cells by hybridization to cDNA pL6-411. Comigration of these three RNAs with 28, 18, and 5.8 S ribosomal RNAs (rRNAs) has suggested the possibility that pL6-411 RNAs are related to ribosomes or ribosome-like structures. Subsequent experiments showed that pL6-411-related RNAs could indeed be found in ribosome-like particles which were indistinguishable from ribosomes when separated on sucrose gradients under native (low salt, isolation of intact ribosomes) or denaturing conditions (detergent, high salt, isolation of ribosome subunits). Furthermore, we demonstrate that pL6-411-related RNAs are cytoplasmic in L6 cells, may be transcribed in nucleoli, and, based on their nucleotide sequence, have the potential of inter- and intramolecular hybridization. Expression of pL6-411 RNAs was also shown in adult as well as in fetal rat tissues after Day 14 of gestation. These above findings provide supportive evidence for the hypothesis that pL6-411 5- and 2-kb RNAs could exist in a subset of ribosomes. These ribosome-like pL6-411 particles nevertheless differ from ribosomes in that their associated RNAs have different nucleotide sequences, are of lower abundance, and are up-regulated later in development than rRNAs. We discuss our results in the context of a postulated ribosome subset containing RNAs other than rRNAs. These ribosome-like particles might be involved in the translational control of ID-positive mRNAs.