After performing a Lys operon mRNA Northern blot and Western blot of the two oth

Question

After performing a Lys operon mRNA Northern blot and Western blot of the two other proteins expressed from the same mRNA, LysD and LysE, your results are as follows

You proceed to determine the sequence of the entire mRNA and its structure is as below

I = Lys repressor gene; D, E, F = Lys operon structural genes; P = Lys promoter; O = Lys operator

You are asked if its possible that there could be more than 3 alternatively spliced variants of the mRNA from the Lys operon that could be reducing the accumulation of the amino acids. What will be your answer?

Explanation / Answer

Thelacoperon (see diagram below) consists ofone regulatory gene (theigene) and three structural genes (z,y, anda). Theigene codes for the repressor of thelacoperon. Thezgene codes for -galactosidase (-gal), which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose. Theygene codes for permease, which increases permeability of the cell to -galactosides. Theagene encodes a transacetylase. During normal growth on a glucose-based medium, thelacrepressor is bound to the operator region of thelacoperon, preventing transcription. However, in the presence of an inducer of thelacoperon, the repressor protein binds the inducer and is rendered incapable of interacting with the operator region of the operon. RNA polymerase isthus able to bind at the promoter region, and transcription of the operon ensues. Thelacoperon is repressed, even in the presence of lactose, if glucose is also present. This repression is maintained until the glucose supply is exhausted. The repression of thelacoperon under these conditions is termedcatabolite repressionand is a result of the low levels of cAMP that result from an adequate glucose supply. The repression of thelacoperon is relieved in the presence of glucose if excess cAMP is added. As the level of glucose in the medium falls, the level of cAMP increases. Simultaneously there is an increase in inducer binding to thelacrepressor. The net result is an increase in transcription from the operon. The ability of cAMP to activate expression from thelacoperon results from an interaction of cAMP with a protein termed CRP (for cAMP receptor protein). The protein is also called CAP (for catabolite activator protein). The cAMP-CRP complex binds to a region of thelacoperon just upstream of the region bound by RNA polymerase and that somewhat overlaps that of the repressor binding site of the operator region. The binding of the cAMP-CRP complex to thelacoperon stimulates RNA polymerase activity 20-to-50-fold.

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