You have studied mutations in a transcription regulatory protein from Bacillus s

Question

You have studied mutations in a transcription regulatory protein from Bacillus subtilis by an in vivo reporter gene assay carried out in Escherichia coli. The cis-regulatory sequence that the protein normally recognizes was inserted upstream of a gene encoding an enzyme that can produce a visible blue product. E. coli cells were then transformed with the reporter construct along with the gene encoding one of the various mutant forms of the transcription regulatory protein. Since the transcription regulatory protein is an inhibitor, the colonies turn blue only when the mutant protein is nonfunctional and unable to inhibit gene expression. Interestingly, you have identified a number of “cold-sensitive” loss-of-function mutations that show the mutant phenotype at 22°C but not at 37°C. Which of the following correctly describes the phenotype of these cold-sensitive mutants? a. Their colonies turn blue at permissive (low) temperatures. b. Their colonies turn blue at permissive (high) temperatures. c. Their colonies turn blue at nonpermissive (low) temperatures. d. Their colonies turn blue at nonpermissive (high) temperatures.

Explanation / Answer

Transcription regulatory protein is an inhibitor and E.Coli colonies turn blue only when the mutant protein is non functional and unable to inhibit gene expression. Many cold sensitive loss of function mutations that show the mutant phenotype at 22 degrees C but not at 37 degrees C are identified.

When a temperature sensitive mutant phenotype is made to grow under permissive temperature, the mutated gene product behaves normally even in the presence of mutant allele. Mutant phenotype is observed only in the non permissive temperature.

Therefore, the answer for this question is option c, their colonies turn blue at nonpermissive (low) temperatures.

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