Editing Lambda phage (section)

====Leftward transcription====
[[File:Phage Lambda int xis Retroregulation.jpg|thumb|right|300px|Diagram showing the retro-regulation process that yields a higher concentration of xis compared to int. The mRNA transcript is digested by bacterial RNase starting from the cleaved hairpin loop at sib.]]
Leftward transcription expresses the ''gam,'' ''xis'', ''bar'' and ''int'' genes.<ref name="src3"/> Gam proteins are involved in recombination. Gam is also important in that it inhibits the host RecBCD nuclease from degrading the 3’ ends in rolling circle replication. Int and xis are integration and excision proteins vital to lysogeny.{{Citation needed|date=November 2023}}

===== Leftward transcription process =====

# Lambda phage inserts chromosome into the cytoplasm of the host bacterial cell.
# The phage chromosome is inserted to the host bacterial chromosome through DNA ligase.
# Transcription of the phage chromosome proceeds leftward when the host RNA polymerase attaches to promotor site ''p''L resulting in the translation of gene ''N.''
## Gene N acts a regulatory gene that results in RNA polymerase being unable to recognize translation-termination sites.<ref name="pmid6241940">{{cite journal | vauthors = Brammar WJ, Hadfield C | title = A programme for the construction of a lambda phage | journal = Journal of Embryology and Experimental Morphology | volume = 83 | issue = Suppl | pages = 75–88 | date = November 1984 | pmid = 6241940 | doi = | url = }}</ref>

===== Leftward Transcription mutations =====
Leftward transcription is believed to result in a deletion mutation of the ''rap'' gene resulting in a lack of growth of lambda phage. This is due to RNA polymerase attaching to pL promoter site instead of the pR promotor site. Leftward transcription results in ''bar''I and ''bar''II transcription on the left operon. Bar  positive phenotype is present when the ''rap'' gene is absent. The lack of growth of lambda phage is believed to occur due to a temperature sensitivity resulting in inhibition of growth.<ref>{{cite journal | vauthors = Guzmán P, Guarneros G | title = Phage genetic sites involved in lambda growth inhibition by the Escherichia coli rap mutant | journal = Genetics | volume = 121 | issue = 3 | pages = 401–409 | date = March 1989 | pmid = 2523838 | pmc = 1203628 | doi = 10.1093/genetics/121.3.401 }}</ref>

===== xis and int regulation of insertion and excision =====
# ''xis'' and ''int'' are found on the same piece of mRNA, so approximately equal concentrations of ''xis'' and ''int'' proteins are produced. This results (initially) in the excision of any inserted genomes from the host genome.
# The mRNA from the ''P<sub>L</sub>'' promoter forms a stable secondary structure with a [[stem-loop]] in the ''sib'' section of the mRNA. This targets the 3' (''sib'') end of the mRNA for RNAaseIII degradation, which results in a lower effective concentration of ''int'' mRNA than ''xis'' mRNA (as the ''int'' cistron is nearer to the ''sib'' sequence than the ''xis'' cistron is to the ''sib'' sequence), so a higher concentrations of ''xis'' than ''int'' is observed.
# Higher concentrations of ''xis'' than ''int'' result in no insertion or excision of phage genomes, the evolutionarily favoured action - leaving any pre-inserted phage genomes inserted (so reducing competition) and preventing the insertion of the phage genome into the genome of a doomed host.