Parameters of gene expression in the bipolar argECBH operon of E. coli K12
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The pattern of divergent transcription of the argECBH cluster of genes previously demonstrated by the hybridization of RNA to the separated strand of a ϕ80 drag transducing phage, is confirmed with the DNA of a set of different λdarg phages.
The accurate determination of argE and argCBH m-RNA levels in different steady states of expression of the arg regulon supports the following conclusions:
The ratio between maximal (derepressed) and minimal (repressed) rates of expression is lower when it is expressed in terms of % hybridizable RNA than in terms of enzyme specific activities. The discrepancy is about 3 fold. Thus in conditions of repression, the cell produces relatively more unused m-RNA than in derepression. Different interpretations of this phenomenon appear possible:
the messenger RNA molecules synthesized in repressed cells could be degraded more rapidly or translated less efficiently than in derepressed cells.
an untranslated segment of the RNA could account for a larger part of the RNA detected in repression than in derepression. These interpretations are not mutually exclusive.
The discrepancy observed between the amplitudes of variation of argE and argCBH expression, expressed in terms of enzyme specific activities, is, in fact, determined at the level of DNA transcription. This provides direct evidence for the occurrence of differential transcription effectiveness in a regulon exhibiting a correlative but not strictly coordinated pattern of enzyme synthesis. This also supports our earlier suggestion regarding the possible complexity of the internal operator region situated between argE and C.
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