Conservation of the ID sequence and its expression as small RNA in rodent brains: analysis with cDNA for mouse brain-specific small RNA.

Two cDNA clones for mouse brain-specific small RNA were isolated, whose sequences turn out to be highly homologous to that of a genomic mouse ID-like clone isolated previously. By use of one of these cDNA clones as a probe, the copy numbers of ID-related sequences in several mammalian species were determined. The ID sequence was found to be conserved in mice, rats, hamsters and guinea pigs, but not in non-rodent species. There were about 3 orders of magnitude of difference in the copy numbers of the gene, i.e. rat 4.0 X 10(4), mouse 4.5 X 10(2), hamster 1.5 X 10(2), and guinea pig less than 50. The gene was transcribed as small RNA in the brain of rodent species. The relative concentration of small RNA in the brain was roughly proportional to the copy number of the ID sequence in mice, hamsters, and probably in guinea pigs also. On the other hand, the amount of the RNA in rat brain was much less than that expected from the large copy number of the gene. On the basis of these observations, the possibility is discussed that most of the ID sequences in the rat are pseudogenes.