SUMMARY: RNA isolated from exponential-phase cultures of was used to titrate denatured DNA over a wide range of RNA: DNA ratios. The following results were obtained: (i) 0.6% of DNA was complementary to purified rRNA and 0.062% was complementary to tRNA. (ii) Under the growth conditions employed (35 °; mean generation time 4 h), unstable RNA accounted for 40% of the rapidly labelled RNA fraction but only 2% of the randomly labelled RNA fraction. The half-life of the unstable RNA was estimated to be 3% of the mean generation time. (iii) A wide variation in the abundance of unstable RNA species was observed; more than 80% of the labelled RNA in both rapidly and randomly labelled unstable RNA fractions was homologous to only 10% of the DNA that was actively transcribed (i.e. 1% of the total DNA). In turn, the actively transcribed DNA comprised only 10% of the total DNA since virtually all the unstable and readily hybridizable RNA fraction (> 99%) from rapidly and randomly labelled RNA would form stable hybrid with it. This indicated that the remaining fraction of the DNA (90%) was infrequently transcribed.


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