Question

Notice that the genes for lysogeny are clustered in an operon on the "left" side of the phage plasmid, while the genes for lysis are clustered in an operon on the "right" side of the plasmid.

At the beginning of each infection, transcription begins at two "early" promoters.

"Leftward" genes

Early promoter P_Lcontrols leftward transcription of early genes, beginning with the N gene. The N protein plays an anti-termination role in transcription from P_L. Leftward transcription is also enhanced by binding of the cII/cIII complex to the P_RE promoter, transcribing the cI gene. The cI protein enhances transcription of "leftward" genes and represses transcription of "rightward" genes. Genes under control of P_L are involved in the integration of viral DNA into the bacterial chromosome.

"Rightward" genes

Early promoter P_R controls rightward transcription of genes, beginning with cro. The cro protein enhances the transcription of the genes that are rightward of the P_R, and prevents transcription of the leftward gene, cI. Genes under the control of P_Rare involved in the synthesis of head and tail proteins as well as proteins that lyse the host cell.

Regulator of Regulator Arci, and int gene of ci gene expression Phage DNA Regulator of replication proteins early genes X repressor cro Stabilizer OP of oil protein N Regulator of Phage- cll late genes recombination Lysis o Pa origin of proteins proteins Excision ase replication (for excision from chromosome) -xis Cutting of Integrase int Nul DNA at (for integration A Terminase cos site for packaging into chromosome att Lamda phage 48,502 nucleotides Nu3 Genes for head proteins F11 and assembly I K L M Genes fortail proteins and assembly

Based on this information, determine the phenotype of each phage mutation. (PLEASE SELECT WHICH GOES INTO WHICH CYCLE)

Answer 1

Answer) To keep the answer simple and easy to understand, let's limit our reasoning for the answers, only to information given in the question.

1) cI^- = No lysogenic cycle; because there is single repression from transcribed rightward gene (as per the given information) which is repression of cI by the cro protein. the absence of cI is like the repressed cI condition and this will demote the transcription of leftward genes and transcription of rightward genes will be automatically enhanced and therefore, the lytic cycle will be favored in the absence of cI.

2) cII^- = No lysogenic cycle; because in the absence of cII protein, there will be no cII/cIII complex formation and hence, no cI transcription. as no cI will be transcribed, there will be no enhancement of the transcription of the leftward genes. this condition will together halt the lysogenic cycle.

3) cro- = No Lytic cycle, because in the absence of "cro protein", there will be no enhancement in rightward transcription of genes. also, there will be no repression of cI protein, in absence of cro protein. this free cI protein will, in turn, repress the transcription of leftward genes.

4) int^- = No Lysogenic cycle; because the "int protein" is responsible for the integration of the lambda phage into the host genome. in absence of the "int protein", the lambda phage DNA can’t be integrated into the host genome and hence, no Lysogenic cycle will be observed.

5) N^- = No Lysogenic cycle; because the N protein acts as an antitermination protein to allow the complete (un-interrupted) transcription of the leftward genes required fo the lysogeny cycle.

6) P_R^- = No Lytic cycle; because the promoter P_R regulates the transcription of the rightward genes required for the lytic cycle.

7) P_L^- = No Lysogenic cycle; because the promoter P_L regulates the transcription of the leftward genes required for lysogeny cycle.