I Lysis P R' Tail assembly protein I. Part of the Qut site resembles the -10 , causing the holoenzyme to pause. The late genes include those for the structural proteins needed to assemble a virus particle. The 12 nucleotides of cohesive ends and process of circularization are shown in Fig. Protein names and their copy numbers in the virion particle are shown. The wild type of this virus has a lifecycle that allows it to either reside within the of its host through or enter into a phase during which it kills and lyses the cell to produce offspring ; mutant strains are unable to lysogenize cells — instead, they grow and enter the lytic cycle after superinfecting an already lysogenized cell.
The left operon is associated with integration and recombination events of lysognic cycle. Phage lambda, which has been most intensively studied, carries a single gene, cI, that encodes a repressor protein. Holliday junction resolvases have been found in the mitochondria of eukaryotes, but nuclear Holliday junction resolvases, if they exist, remain to be discovered and elaborated. This results initially in the excision of any inserted genomes from the host genome. Genetic Map of Phage Lambda 5. Int-mediated recombination requires that both participants have an att site.
At first, these express the N and cro genes, producing N, Cro and a short inactive protein. LexA expression leads to inhibition of various genes including LexA. The N and Q genes are also required in plaque formation, in the absence of which the number of phage particles would be less but not zero. Morphology of Phage Lambda 2. This prophage may enter the lytic cycle when the lysogen enters a stressed condition. The system allows phage λ to make a fast exit in times of stress to the bacterial host.
It is interesting to note that repressor protein coded by cI regulates its own synthesis also by working as an activator, so that if repressor is inactivated, its further synthesis does not take place and the phage is forced to enter a lytic cycle. Fig B: Early genes are continued to be expressed. The head is joined to a non-contractile 180 µm long tail by a connector. Repressor also inhibits transcription from the P L promoter. The N protein stimulates b delayed-early and the Q protein stimulates c late gene expression. The detail of this competition is not yet clear but the environmental factors influence the result of this race for choice of the two cycles. Also the tail consists of about 35 stacked discs or annuli.
The repressor is now synthesized under the influence of P Mdue to the positive control of repressor already synthesized under the control of promoter P E , which binds on O R which also contains the promoter P R. This complex skips through most termination sequences. When lysogeny is established, only sites 1 and 2 are occupied but site 3 is not occupied. Early gene expression ends after sigma factor is produced. This leads to the lysogenic lifestyle. The three red genes code for three proteins at normal frequency for general recombination. The cleavage releases the C-terminal domains, so that N-terminal domains now do not have sufficient affinity to remain attached to the operator.
The sequence of the bacterial att site is called attB, between the gal and bio operons, and consists of the parts B-O-B', whereas the complementary sequence in the circular phage genome is called attP and consists of the parts P-O-P'. Lysogeny through establishment and maintenance of repressor promoters P Eand P M In phage lambda λ , a very delicate balance is maintained between lytic cycle and lysogeny, with a very sophisticated and intricate circuit of events, which will be briefly described in this section. Note that NusA can stimulate the activity of the Q protein. This is a key element in the differentiation between lytic and temperate growth of the phage. . When transcribed, each sequence forms a hairpin loop structure that the N protein can bind to.
Fig B: Early genes are separated form next expressed genes by terminator site. The lytic cycle ends with lysis of E. S is a , a small membrane protein that, at a time determined by the sequence of the protein, suddenly makes holes in the membrane. It is therefore the only protein expressed by lysogenic phage. B Simplified symbolic version of the kinetic model in A.
This is a phenomenon called long-range cooperativity. Int and xis are integration and excision proteins vital to lysogeny. In each case, site 1 is closest to its promoter P Rand P L and site 3 is farthest. It ends in a fiber. These enzymes and the λ enzyme differ from RuvC in that they can cleave three-strand junctions and Y-junctions in addition to four-strand junctions. Synthesis and role of Cro protein in prevention of repressor maintenance both through Pm directly due to binding at O land O r and P e indirectly due to switching off the delayed early genes , thus leading to lysis. The repressor functions as dimers for maintenance of lysogeny, and at low concentration it binds only to sites O R1and O L1 , while with increasing concentration, the repressor dimers can also bind to sites 2 O R2 and O L2 and 3 O R3 and O L3.