Mutations

Mutations

	Mutations -The linkage group of E. coli is calibrated from 0 to 100 minutes. Among the operons of E. coli are the lactose (lac) operon and the galactose (gal) operon. The gal operon consists of the operator-promoter region (OP) and three structural genes for three enzymes: Galactokinase gene - galK Transferase gene - galT Epimerase gene - galE
The gal U gene coding for uridine diphosphoglucose pyrophosphorylase is located at minute 27, and is Dot a part of the gal operon. The lactose (lac) operon consists of a promoter (P), an operator (0) and three structural genes: β- galactosidase gene - Z lac permease gene - Y Thiogalactoside transacetylase gene-A
When an IS element is inserted into a gene, it abolishes the function of the gene, and thus causes a mutation. Many of the spontaneously polar mutations of the gal operon of E. coli are caused by the insertion of IS elements. The location of the IS sites is not random, and there appear to be certain favoured regions, called hot spots, for IS-induced mutations. The IS elements thus appear to insert themselves at specific sites within the genes.
With respect to the polarity of mutations, two classes of mutations have been found in the gal operon of E. coli. One class, consisting of insertions in the gal OP region, causes a reduction in the synthesis of the three enzymes (galactokinase, transferase and epimerase) of the gal operon. The other class causes almost a complete absence of epimerase synthesis. Thus there appear to be two independent but very close insertion sites for IS elements in the gal operon. The gal3 mutation is caused by the insertion of a 1100-1200 base pair IS2 element in the OP region of the gal operon. IS2 turns off gene expression in a reversible manner.
	The mutations produce three kinds of spontaneous gal + revertants. (i) Precise excision of the insertion results in a stable, inducible reversion, and the gal+ base sequence is restored. (ii) Imprecise excision of the insertion results in a stable!, constitutive reversion. (iii) Tandem duplication of the gal operon causes an unstable constitutive,reversion. The IS2 element can (i) turn off gene expression in a reversible manner, (ii) act as a 'hot spot' for recombination and for linking unrelated genomes and (iii) act as a preferred site for deletion.

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