An Hfr strain is used to map three genes in an interrupted mating experiment. The cross is Hfr/a⁺b⁺c⁺ rif x F⁻/a⁻b⁻c⁻ rif^T (No map order is implied in the listing of the alleles; rif^T is resistance to the antibiotic rifampicin.) The a⁺ gene is required for the biosynthesis of nutrient A, the b⁺ gene for nutrient B, and the c⁺ gene for nutrient C. The minus alleles are auxotrophs for these nutrients. The cross is initiated at time = 0, and at various times, the mating mixture is plated on three types of medium. Each plate contains minimal medium (MM) plus rifampicin plus specific supplements that are indicated in the following table. (The results for each time interval are shown as the number of colonies growing on each plate.)

Based on these data, determine the approximate location on the chromosome of the a, b, and c genes relative to one another and to the F factor.

An Hfr strain with the genotype cys⁺ lue⁺ met⁺ str^S is mated with an F^- strain carrying the genotype cys⁻ lue⁻ met⁻ strᴿ. In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows:

Medium 1: Minimal medium plus leucine, methionine, and streptomycin

Medium 2: Minimal medium plus cysteine, methionine, and streptomycin

Medium 3: Minimal medium plus cysteine, leucine, and streptomycin

What donor gene is the selected marker in each medium?

Synthesis of the amino acid histidine is a multistep anabolic pathway that uses the products of 13 genes (hisA to hisM) in E. coli. Two independently isolated his^- E. coli mutants, designated his1⁻ and his2⁻ are studied in a conjugation experiment. A his⁺ F' donor strain that carries a copy of the hisJ gene on the plasmid is mated with a his1⁻ recipient strain in Experiment 1 and with a his2⁻ recipient in Experiment 2. The exconjugants are grown on plates lacking histidine. Growth is observed among the exconjugants of Experiment 2 but not among those of Experiment 1.

What is the genotype of exconjugants in Experiment 2?

Synthesis of the amino acid histidine is a multistep anabolic pathway that uses the products of 13 genes (hisA to hisM) in E. coli. Two independently isolated his^- E. coli mutants, designated his1⁻ and his2⁻ are studied in a conjugation experiment. A his⁺ F' donor strain that carries a copy of the hisJ gene on the plasmid is mated with a his1⁻ recipient strain in Experiment 1 and with a his2⁻ recipient in Experiment 2. The exconjugants are grown on plates lacking histidine. Growth is observed among the exconjugants of Experiment 2 but not among those of Experiment 1.

Why is growth observed in Experiment 2 but not in Experiment 1?

An Hfr strain is used to map three genes in an interrupted mating experiment. The cross is Hfr/a⁺b⁺c⁺ rif x F⁻/a⁻b⁻c⁻ rif^T (No map order is implied in the listing of the alleles; rif^T is resistance to the antibiotic rifampicin.) The a⁺ gene is required for the biosynthesis of nutrient A, the b⁺ gene for nutrient B, and the c⁺ gene for nutrient C. The minus alleles are auxotrophs for these nutrients. The cross is initiated at time = 0, and at various times, the mating mixture is plated on three types of medium. Each plate contains minimal medium (MM) plus rifampicin plus specific supplements that are indicated in the following table. (The results for each time interval are shown as the number of colonies growing on each plate.)

Can the location of the rif gene be determined in this experiment? If not, design an experiment to determine the location of rif relative to the F factor and to gene b.

An Hfr strain is used to map three genes in an interrupted mating experiment. The cross is Hfr/a⁺b⁺c⁺ rif x F⁻/a⁻b⁻c⁻ rif^T (No map order is implied in the listing of the alleles; rif^T is resistance to the antibiotic rifampicin.) The a⁺ gene is required for the biosynthesis of nutrient A, the b⁺ gene for nutrient B, and the c⁺ gene for nutrient C. The minus alleles are auxotrophs for these nutrients. The cross is initiated at time = 0, and at various times, the mating mixture is plated on three types of medium. Each plate contains minimal medium (MM) plus rifampicin plus specific supplements that are indicated in the following table. (The results for each time interval are shown as the number of colonies growing on each plate.)

What is the purpose of rifampicin in the experiment?

Look closely at the consolidated Hfr map and the data used to build the map on page 261. Suppose a fifth Hfr strain had the F factor inserted exactly halfway between cysE and leuU and had an orientation that was the same as that of Hfr 1. List the order of gene transfer for the first six genes transferred by this Hfr and the number of minutes of conjugation at which each gene is expected to be seen.

Bacterial conjugation, mediated mainly by conjugative plasmids such as F, represents a potential health threat through the sharing of genes for pathogenicity or antibiotic resistance. Given that more than 400 different species of bacteria coinhabit a healthy human gut and more than 200 coinhabit human skin, Francisco Dionisio [(2002) Genetics 162:1525 1532] investigated the ability of plasmids to undergo between-species conjugal transfer. The following data are presented for various species of the enterobacterial genus Escherichia. The data are presented as 'log base 10' values; for example, -2.0 would be equivalent to 10⁻² as a rate of transfer. Assume that all differences between values presented are statistically significant.

What is the significance of these findings in terms of human health?

Bacterial conjugation, mediated mainly by conjugative plasmids such as F, represents a potential health threat through the sharing of genes for pathogenicity or antibiotic resistance. Given that more than 400 different species of bacteria coinhabit a healthy human gut and more than 200 coinhabit human skin, Francisco Dionisio [(2002) Genetics 162:1525 1532] investigated the ability of plasmids to undergo between-species conjugal transfer. The following data are presented for various species of the enterobacterial genus Escherichia. The data are presented as 'log base 10' values; for example, -2.0 would be equivalent to 10⁻² as a rate of transfer. Assume that all differences between values presented are statistically significant.

In what species is within-species transfer most likely? In what species pair is between-species transfer most likely?