[Determination of the upper limit of the value for rat genome expression]

Nuclear RNA was isolated from the majority of adult rat tissues, and from the whole embryos at three different stages of gestation. 13 different mRNA preparations were mixed and the sequence complexity of this total RNA (sigma RNA) was measured by RNA-driven hybridization to [125I]DNA under conditions of maximal stringency. With unique DNA as a probe, the complexity of sigma RNA was equivalent (by assuming asymmetric transcription) to 55.4% of the single-copy genome, corresponding to enough information for 2.2 . 10(5) different 4.5 k.b. genes. With DNA probe containing both unique and middle-repetitive sequences (C0t greater than or equal to 0.02), the complexity of sigma RNA was equivalent to 67.6% of the genome. Nearly all middle-repetitive DNA is represented (by assuming asymmetric transcription) in embryos nuclear RNA. The value of genetic complexity of sigma RNA (55.4 and 67.6%) represents the highest published value of RNA complexity and approach the maximal theoretical capacity of the mammalian genome. We conclude that despite the dramatic increase of the genome size in higher animals, the relative values of their genetic information expression (at the level of nuclear RNA) is comparable with that in lower eukaryotes and bacteria.