A 34-year-old woman comes to the office due to dysuria. The patient has a history of recurrent urinary tract infections. A urine sample is collected and sent for culture. Gram-negative bacteria isolated from the urine are found to form pink colonies on lactose-containing MacConkey agar. Several days later, bacterial isolates from a second urine sample are found to form white colonies when plated on the same type of medium. Genetic analysis shows that the more recent isolates have a single nucleotide deletion within the lac operon DNA sequence. This genomic change is most consistent with which of the following?

Question

A 34-year-old woman comes to the office due to dysuria.  The patient has a history of recurrent urinary tract infections.  A urine sample is collected and sent for culture.  Gram-negative bacteria isolated from the urine are found to form pink colonies on lactose-containing MacConkey agar.  Several days later, bacterial isolates from a second urine sample are found to form white colonies when plated on the same type of medium.  Genetic analysis shows that the more recent isolates have a single nucleotide deletion within the lac operon DNA sequence.  This genomic change is most consistent with which of the following?

Quizplus · Accepted Answer

11ec160b_376f_fc27_9249_73f97a13c350_MD0001_00 Organisms that ferment lactose appear pink on MacConkey agar whereas lactose non-fermenters form white colonies. In this case, the single nucleotide deletion within the lac operon DNA sequence of the newer isolates impairs lactose metabolism most likely by preventing the formation of a required enzyme. Because the mutation is a single base deletion, it will cause a frameshift mutation if it occurs within the coding region (exons) of the gene. Frameshift mutations are caused by deletion or insertion of any number of nucleotides that are not multiples of 3. The result is a change in the reading frame during protein translation, which causes the production of an entirely different protein that is often shorter than the original due to formation of a premature stop codon. The other mutations listed are all examples of point mutations (ie, single base substitutions) that can lead to a variety of changes in the coded protein. (Choices A and C) Missense mutations are single base substitutions that result in the placement of an incorrect amino acid in a protein sequence. These mutations occur within the coding region of a gene and do not alter protein length. A conservative mutation is a type of missense mutation where an amino acid is replaced with another that has similar biochemical characteristics. Conservative mutations generally preserve protein function unless the mutation occurs in a critical region (eg, active site of enzyme). (Choice D) Nonsense mutations introduce a premature stop codon in a coding region, resulting in the production of a truncated protein. The mutation described is not a nonsense mutation because it was caused by a single base deletion (not a base substitution). (Choice E) A silent mutation is a single base substitution within a codon that does not change the amino acid (due to codon redundancy). Silent mutations have no effect on protein formation or function. Educational objective: A frameshift mutation occurs with the deletion/addition of a number of bases not divisible by 3 in the coding region of a gene. Frameshift mutations alter the reading frame of the genetic code, dramatically changing the protein structure and often resulting in the formation of a premature stop codon.

A 34-Year-Old Woman Comes to the Office Due to Dysuria