Merodiploid Interpretation 1. Lac Complementation; m] and m2 are in different gene 2. Lac No complementation; mJ and m2 are in the same gene Complementation of two recessive mutations. One mutation is in the chromosome, and the other is in a prime factor. If the two mutations complement each other the cells will be lact and will grow with lactose as the sole carbon and energy source The mutations will not complement if they are in the same gene or if one affects a regulatory site or is polar
Complementation of two recessive mutations. One mutation is in the chromosome, and the other is in a prime factor. If the two mutations complement each other, the cells will be Lac+ and will grow with lactose as the sole carbon and energy source. The mutations will not complement if they are in the same gene or if one affects a regulatory site or is polar
cis-Acting lac mutations Not all lac mutations affect diffusible gene products and can be complemented. Immediately adjacent to the lacz mutations are other lac mutations that are much rarer and have radically different properties. These mutations cannot be complemented to allow the expression of the lac genes on the same dna even in the presence of good copies of the lac genes Recessive mutations that cannot be complemented are cis acting and presumably affect a site on dna rather than a diffusible gene product like rna or protein
• cis-Acting lac Mutations • Not all lac mutations affect diffusible gene products and can be complemented. Immediately adjacent to the lacZ mutations are other lac mutations that are much rarer and have radically different properties. These mutations cannot be complemented to allow the expression of the lac genes on the same DNA, even in the presence of good copies of the lac genes. Recessive mutations that cannot be complemented are cis acting and presumably affect a site on DNA rather than a diffusible gene product like RNA or protein
To show that a lac mutation is cis-acting, i. e. affects only the expression of genes on the same dna where it occurs, we could introduce an F'factor containing the potential cis-acting lac mutation into cells containing either a lacz or a lacr mutation in the chromosome. Trans-acting gene products encoded by the fi factor lacz or lacy genes would complement the chromosomal lacy or lacz mutations respectively However, if the resulting phenotypes are Lac", the lac mutation in the Ffactor must prevent expression of both LacZ and LacY proteins from the f factor. The mutation in the f factor is therefore cis-acting One type of the cis-acting lac mutations affects lacp and is a promoter mutation that prevents transcription of the lacz and lacy genes by changing the binding site for RNa polymerase on the DNA. another type of cis-acting mutation is a polar mutation in lacz that prevents the transcription of the downstream lacy gene
• To show that a lac mutation is cis-acting, i.e., affects only the expression of genes on the same DNA where it occurs, we could introduce an F' factor containing the potential cis-acting lac mutation into cells containing either a lacZ or a lacY mutation in the chromosome. Trans-acting gene products encoded by the F' factor lacZ or lacY genes would complement the chromosomal lacY or lacZ mutations, respectively. However, if the resulting phenotypes are Lac- , the lac mutation in the F' factor must prevent expression of both LacZ and LacY proteins from the F factor. The mutation in the F factor is therefore cis-acting. • One type of the cis-acting lac mutations affects lacp and is a promoter mutation that prevents transcription of the lacZ and lacY genes by changing the binding site for RNA polymerase on the DNA. Another type of cis-acting mutation is a polar mutation in lacZ that prevents the transcription of the downstream lacY gene
Merodiplo d Phenaty Interpretation Complementation mj and m2 are in different genes No complementation m] and m2 are in the same gene The lacp mutations cannot be complemented and are cis acting. A lacp mutation in the prime factor will prevent expression of any of the other lac genes on the prime factor, so that a lac mutation in the chromosome will not be complemented. Partial diploid cells will be L:
The lacp mutations cannot be complemented and are cis acting. A lacp mutation in the prime factor will prevent expression of any of the other lac genes on the prime factor, so that a lac mutation in the chromosome will not be complemented. Partial diploid cells will be Lac-
Regulation by attenuation of Transcription In the above examples, the transcription of an operon is regulated through the initiation of rna synthesis at the promoter of the operon. However, this is not the only known means of regulating operon transcription. Another mechanism is the attenuation of transcription. Unlike repressors and activators, which turn on or off transcription from the promoter the attenuation mechanism works by terminating transcription-which begins normally--before the rna polymerase reaches the first structural gene of the operon. The classic examples of regulation by attenuation are the his and trp operons of E. coli. Closely related mechanisms regulate such E coli biosynthetic operons as leucine (leu phenylalanine(phe), threonine(thr), and isoleucine-valine (ilv) and the bacillus subtilis trna synthetase genes
• Regulation by Attenuation of Transcription • In the above examples, the transcription of an operon is regulated through the initiation of RNA synthesis at the promoter of the operon. However, this is not the only known means of regulating operon transcription. Another mechanism is the attenuation of transcription. Unlike repressors and activators, which turn on or off transcription from the promoter, the attenuation mechanism works by terminating transcription—which begins normally—before the RNA polymerase reaches the first structural gene of the operon. The classic examples of regulation by attenuation are the his and trp operons of E. coli. Closely related mechanisms regulate such E. coli biosynthetic operons as leucine (leu), phenylalanine (phe), threonine (thr), and isoleucine-valine (ilv) and the Bacillus subtilis tRNA synthetase genes