Info

A mutant strain of E. coli produces p-galactosidase in the presence and in the absence of lactose. Where in the operon might the mutation in this strain occur?

For E. coli strains with the following lac genotypes, use a plus sign ( + ) to indicate the synthesis of p-galactosidase and permease and a minus sign ( —) to indicate no synthesis of the enzymes.

Lactose absent

Lactose present

Genotype of strain lacI+ lacP+ lacO+ lacZ+ lacY+ lacI" lacP+ lacO+ lacZ+ lacY+ lacI+ lacP+ lacOc lacZ+ lacY+ lacI" lacP+ lacO+ lacZ+ lacY~ lacI" lacP" lacO+ lacZ+ lacY+

lacI+ lacP+ lacO+ lacZ" lacY+/ lacI" lacP+ lacO+ lacZ+ lacY~

ß-Galactosidase Permease ß-Galactosidase Permease

(continued on p. 469)

*23. (continued) Lactose absent Lactose present

Genotype of strain p-Galactosidase Permease P-Galactosidase Permease

lacI

lacP4

lacOc

lacZ4

lacY4 /

lacI+

lacP4

lacO4

lacZ~

lacY-

lacI~

lacP4

lacO4

lacZ4

lacY-/

lacI4

lacP~

lacO4

lacZ~

lacY4

lacI+

lacP~

lacOc

lacZ-

lacY4 /

lacI~

lacP4

lacO4

lacZ4

lacY-

lacI4

lacP4

lacO4

lacZ4

lacY4 /

lacI4

lacP4

lacO4

lacZ4

lacY4

lacIs

lacP4

lacO4

lacZ4

lacY-/

lacI+

lacP4

lacO4

lacZ~

lacY4

lacIs

lacP~

lacO4

lacZ-

lacY4 /

lacI4

lacP4

lacO4

lacZ4

lacY4

24. Give all possible genotypes of a lac operon that produces p-galactosidase and permease under the following conditions. Do not give partial diploid genotypes.

Lactose absent Lactose present

P-Galactosidase Permease P-Galactosidase Permease

*25. Explain why mutations in the lacI gene are trans in their effects, but mutations in the lacO gene are cis in their effects.

*26. The mmm operon, which has sequences A, B, C, and D, encodes enzymes 1 and 2. Mutations in sequences A, B, C, and D have the following effects, where a plus sign ( + ) = enzyme synthesized and a minus sign ( - ) = enzyme not synthesized.

Mmm absent

Mmm present

Mutation Enzyme Enzyme Enzyme Enzyme in sequence 12 12

(a) Is the mmm operon inducible or repressible?

(b) Indicate which sequence (A, B, C, or D) is part of the following components of the operon:

Regulator gene _

Promoter _

Structural gene for enzyme 1 _

Structural gene for enzyme 2 _

*27. Listed in parts a through g are some mutations that were found in the 5' UTR region of the trp operon of E. coli. What would the most likely effect of each of these mutations be on the transcription of the trp structural genes?

(a) A mutation that prevented the binding of the ribosome to the 5' end of the mRNA 5' UTR

(b) A mutation that changed the tryptophan codons in region 1 of the mRNA 5' UTR into codons for alanine

(c) A mutation that created a stop codon early in region 1 of the mRNA 5' UTR

(d) Deletions in region 2 of the mRNA 5' UTR

(e) Deletions in region 3 of the mRNA 5' UTR

(f) Deletions in region 4 of the mRNA 5' UTR

(g) Deletion of the string of adenine nucleotides that follows region 4 in the 5' UTR

28. Some mutations in the trp 5' UTR region increase termination by the attenuator. Where might these mutations occur and how might they affect the attenuator?

29. Some of the mutations mentioned in Question 28 have an interesting property. They prevent the formation of the antiterminator that normally takes place when the tryptophan level is low. In one of the mutations, the AUG start codon for the 5' UTR peptide has been deleted. How might this mutation prevent antitermination from occurring?

30. Several examples of antisense RNA regulating translation in bacterial cells have been discovered. Molecular geneticists have also used antisense RNA to artificially control transcription in both bacterial and eukaryotic genes. If you wanted to inhibit the transcription of a bacterial gene with antisense RNA, what sequences might the antisense RNA contain?

*31. What would be the effect of deleting the Sxl gene in a newly fertilized Drosophila embryo?

32. What would be the effect of a mutation that destroyed the ability of poly(A)-binding protein (PABP) to attach to a poly(A) tail?

0 0

Post a comment