Chapter 19, Problem 28CONQ

Summary Introduction

To analyze:

Consider an operon in the bacterium Salmonella typhimuriumwith the following arrangement:

The promoter for this operon is contained within a transposable element. gene H $2$ encodes a protein that is a part of the bacterial flagellum. Thegene rH $1$ encodes a repressor protein which functions to repress H $1$ gene. This gene is located at another location in the chromosome The H $1$ gene is also involved in encoding a flagellar protein. When the promoteris found in the arrangement shown in the given figure, the $\text{H2/rH1}$ operon isturned on. This results in flagella that contain H $2$ protein. H $1$ protein is not made because rH $1$ repressor prevents the transcription of H $1$ gene. At a frequency of approximately $\text{1}$ in $\text{10000}$, this strain of bacterium can “switch” its expression so that H $2$ is turned off and H $1$ is turned on. Bacteria that have H $1$ turnedon and H $2$ turned off can also switch back to having H $2$ turnedon and H $1$ turned off. This switch also occurs at a frequency ofabout $\text{1}$ in $\text{10000}$. Considering transposonsand recombination, explain how switching occurs in Salmonella typhimurium.

Introduction:

In recombination, pairing of identical or similar sequences, followed by crossing over and the resolution of the interwined helices takes place. Sometimes, the direct repeats or inverted repeats within a single transposon element can align and undergo homologous recombination. The consequences of recombination of direct repeats and recombination of inverted repeats are different.