We now examine how an infecting lambda phage eventually shuts down gene activity of all but three of its genes when it lysogenizes a cell. The beginning steps are the same as in a lytic infection: DNA is injected into the cell, and the N and Cro proteins, which are sometimes called the immediate early genes, begin to accumulate. Then the remainder of the proteins synthesized under control of the p_L and p_R promoters, including CII and CIII, begin to be synthesized. These latter proteins are crucial to the lysogenic response in three ways. First, they are necessary for the initial synthesis of CI; second, they are necessary for adequate synthesis of the phage protein required for integration of phage DNA into the host DNA, the Int protein; and third, they delay or reduce the expression of the Q protein that turns on late protein synthesis.

The CII protein activates transcription from three phage promoters p_RE, p_I, and p_AQ. The CIII protein works with CII somehow to protect CII from the action of proteases within the cell. The sensitivity of CII to proteases and the protection provided by CIII generate a response highly sensitive to the physiological state of the cell. It is this response that determines whether or not a cell will proceed down the lytic or the lysogenic developmental pathways. The promoter p_RE is located to the right of cro and is oriented in the direction opposite to p_R. Thus the initial part of the RNA synthesized under control of this promoter is anti-cro messenger and likely reduces Cro protein synthesis. The remainder of the messenger encodes repressor protein, the CI gene product, as well as two other proteins in this operon, RexA and RexB. The p_I promoter lies partly in the xis gene and is just in front of the int gene. The p_AQ promoter is located within the Q gene and oriented in the opposite direction. Thus, the p_AQ promoter specifies the synthesis of antimessenger that reduces Q protein synthesis until CII is gone.

In infected cells that will become lysogens, repressor soon reaches a level sufficient to bind to the operators that overlap p_L and p_R. This binding shuts off synthesis of N, Cro, and the other early gene products. Hence, although a little Exo, P, γ, O, P, and Q have accumulated, their levels are insufficient to sustain growth. Lambda destined to become a lysogen may even undergo several rounds of DNA replication, but then the early genes are shut off and the instability of O prevents further development. The Int protein, which also has been synthesized at high rates under control of p_I, participates with a number of host proteins in the integration of one copy of the lambda DNA into the host.

After high levels of lambda repressor have effectively shut off the phage early promoters, four phage promoters can still produce functional messenger, p_RM, p_RE, p_I, and p_AQ. However as CII and CIII lose activity and are diluted away with cell growth, p_RE, p_I, and p_AQ shut down and leave only the promoter for the maintenance of repressor synthesis, p_RM, active. The program of shut-down occurs whether or not lambda has been successful in integrating itself into the host chromosome. If integration has occurred, then the lambda DNA is replicated by the host machinery upon each synthesis cycle and each daughter cell inherits a copy of the lambda genome and is lysogenic. If integration has not occurred, then lambda is passed to only one daughter and soon is effectively lost by dilution.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم العلاقات العامة يختتم فعاليات محطات فتية الكفيل الثقافية في محافظة نينوى

ما الخدمات التي قدّمها موكب أشبال الطفّ لزائري عاشوراء؟

العتبة العباسية المقدسة تنظّم مسيرات عزائية لإحياء ذكرى عاشوراء في نينوى

قسم الشؤون الفكرية يحيي ذكرى استشهاد الإمام الحسين (عليه السلام) في القارة الإفريقية

المزيد

الآخبار الصحية

تناول الجبن.. مفيد للجسم أم خطر يهدد صحتك؟

طبيبة أسنان تحذر.. لا تتجاهل هذه العلامة بعد التنظيف

يجب أن تأكل المزيد من الشمام هذا الصيف.. إليك السبب ؟

تأثير صحي كبير.. كم بيضة يجب أن تتناول يوميا؟

المزيد

الآخبار التكنلوجية

أداء خلايا البيروفسكايت الشمسية تحت الماء قد يزيد كفاءة تحويل الكهرباء

مسافة الأمان: أساس القيادة الآمنة وتجنب الحوادث

هل القيادة السريعة تعني استهلاك اكثر للوقود؟

لماذا يعتبر كوكب المريخ غير صالح للحياة؟

المزيد

مواضيع ذات صلة

The Need for and the Realization of Hair-Trigger Induction

Site for Cro Repression and CI Activation

N Protein and Antitermination of Early Gene Transcription

Spontaneous Mutations in bacteria

Mechanisms of Gene Transfer in Bacteria

Phage Replication

Genotoxicity Through Potential Off-target Nuclease - Induced Double - Strand Breaks

A Broad Range of Genomic Changes by Homologous Recombination–Mediated Genome Editing

Early Transcription of Genes N and Cro

Lambda’s Relatives and Lambda Hybrids

The Genetic Structure of Lambda

The Physical Structure of Lambda