Lecture 5 - Bacteriophage

Question 1

Briefly describe the T7 lytic growth cycle on a genetic level.

Answer 1

The early RNA is transcribed by host RNA polymerase, which transcribes a viral RNA polymerase. The viral polymerase transcribes the late RNA, which also includes gene product 2 that inhibits host RNA polymerase. Therefore, the bacterial cell’s machinery becomes dedicated to making late RNA gene products.

Question 2

Briefly describe the lambda bacteriophage lytic growth cycle.

Answer 2

Host RNA polymerase binds to a few phage promoters and transcribes up to the initial termination sequences. This continues until enough N protein is made. N protein is an anti-terminator that allows for read-through of the termination sites. As a result of the read-through, Q protein is made that commits the cell to the lytic phase.

Question 3

Briefly describe the lambda bacteriophage lysogenic growth cycle.

Answer 3

Initial steps are the same as lytic phase.

CII is a protein that is transcribed in a region near Q. If the bacteria is in a nutrient-rich environment there will be bacterial protease in the cytosol that degrades CII. If not, CII will be active, which allows the host RNA polymerase to recognize promoters that result in CI transcription. CI binds to PL and PR to stop transcription of int, xis, and late genes. CI also binds to PRM that result in continued CI transcription.

Question 4

Briefly describe the induction and excision phases of a lysogenic phage.

Answer 4

DNA damage is sensed by RecA. RecA sends a signal to LexA (that normally stops the SOS genes from repairing DNA) to auto-degrade. Similarly, RecAs cause auto-degradation of CI, allowing the phage ins, xis, and late genes to start their transcription. ins + xis = excision.

Question 5

What can imprecise excision result in?

Answer 5

When a piece of bacterial DNA is accidentally excised out with the phage DNA and inserted into a new bacterial host, the host may gain a new gene. This is called specialized transduction, or lysogenic conversion. Bacteria may gain toxin genes this way as in the case of the strain involved in the Seattle Jack in the Box E. Coli outbreak.