Histidine operon polarity

In Section III,B, the phenomenon of polarity was discussed in relation to its implications for a polycistronic mRNA for the histidine operon. For the deduction of the presence of a polycistronic mRNA, it was not necessary to know the specific mechanism by which polar mutations exerted their effect. However, a great deal of effort has been expended in trying to understand this mechanism, and these studies have produced a greater understanding of the translation process. 

In an attempt to study natural polarity, mutations in different parts of the histidine operon were isolated which cause polarity, but do not result in a histidine requirement [70]. The very existence of these modulation mutants shows that it is possible for the cell to optimize the gradient of production of the enzymes of an operon. The analysis of one of these mutants is discussed in the following section. 

According to their theory, as the polycistronic messenger RNA moves in relation to the polysome system, the velocity of protein synthesis in its various parts is slowed. They postulated that the sequence of the genes in the histidine operon (which does not correspond to the biochemical sequence of reactions) is connected with the number of molecules of each enzyme synthesized. By analyzing the frequency of mutations of polarity, they concluded that many triplets (of the 64 possible) can retard the transcription and translation of information. The essence of the matter is that if any nucleotide triplet (codon) XYZ requires an anticodon in the molecules of acceptor sRNA for itstranslationinto a protein "text," a lowered content of this fraction of sRNA with the corresponding anticodon may act as modulator of the velocity of translation, which is reduced at this locus in connection with a decrease in the number of codon-anticodon interactions. 

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