Abstract
The enzyme-forming capacity of lac messenger RNA in Escherichia coli was followed in vivo after induction with isopropyl thiogalactoside by measuring the difference between the amount of β-galactosidase at the moment of de-induction and after full expression of preformed messenger. It was shown that rifampicin stops the process at the same step as de-induction by inducer removal and is without detectable effect on subsequent expression of preformed messenger RNA. lac Messenger RNA initiation seems to occur randomly in time without detectable periodicity. The rate of elongation of messenger RNA was measured by the combined use of rifampicin and of actinomycin D. The synthesizing capacity is due to four classes of messenger RNA: terminated or non-terminated, capable or not capable of initiating new peptide chains. A study of the ratios of these four classes of messenger during steady-state of induced synthesis gave results consistent with a model where inactivation of messenger RNA occurs randomly in time at the initiating end and does not affect protein synthesis by pre-engaged ribosomes. Moreover, the times of transcription and of translation of a given segment of messenger RNA are equal within experimental error. The changes occurring in the synthesizing capacity of messenger RNA were examined under a variety of conditions where translation was inhibited. Initiation and elongation of messenger RNA were not inhibited by 5-methyl-tryptophan, by multiple amino-acid starvation or by puromycin. Thus, there appears to be no obligatory coupling between transcription and translation. However 5-methyltryptophan and multiple amino-acid starvation extended the transcription time while puromyein did not. In contrast, multiple amino-acid starvation and puromycin shortened the half-life of messenger RNA while 5-methyl-tryptophan was without effect on this parameter.
Published Version
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