Abstract
The CII protein of temperate coliphage 186, like the unrelated CII protein of phage λ, is a transcriptional activator that primes expression of the CI immunity repressor and is critical for efficient establishment of lysogeny. 186-CII is also highly unstable, and we show that in vivo degradation is mediated by both FtsH and RseP. We investigated the role of CII instability by constructing a 186 phage encoding a protease resistant CII. The stabilised-CII phage was defective in the lysis-lysogeny decision: choosing lysogeny with close to 100% frequency after infection, and forming prophages that were defective in entering lytic development after UV treatment. While lysogenic CI concentration was unaffected by CII stabilisation, lysogenic transcription and CI expression was elevated after UV. A stochastic model of the 186 network after infection indicated that an unstable CII allowed a rapid increase in CI expression without a large overshoot of the lysogenic level, suggesting that instability enables a decisive commitment to lysogeny with a rapid attainment of sensitivity to prophage induction.
Highlights
The genetic and molecular machinery that governs the lytic/lysogenic life cycle decision of temperate phage has proven to be fertile ground for analysing the operation of genetic switches [1,2]
The CII protein is a critical component of the decision-making circuit, being a pro-lysogenic factor necessary for establishing lysogeny after infection. CII primes production of the lysogenic repressor CI, inhibits expression of late lytic genes and activates expression of the integrase gene [1,6] (Figure 1A). CII is encoded on the lytic transcript, creating a delayed negative feedback on lytic development. CII is rapidly degraded in vivo by the protease FtsH, and is protected from FtsH by the CIII protein [7,8,9]
The complexity of the phage genetic network does not allow this effect to be directly linked to the behaviour of the core developmental switch, since CII is active at pro-lysogenic promoters paQ and pI [17], outside the switch region (Figure 1A)
Summary
The genetic and molecular machinery that governs the lytic/lysogenic life cycle decision of temperate phage has proven to be fertile ground for analysing the operation of genetic switches [1,2]. The developmental switch from 186-related temperate phage P2 lacks a CII-like factor and the associated delayed negative feedback [13] (Figure 1C), demonstrating that a CII-like factor is not a necessary component of a genetic switch governing two alternative, stable states. The complexity of the phage genetic network does not allow this effect to be directly linked to the behaviour of the core developmental switch (cI-pRM-pR-cro-pRE-cII), since CII is active at pro-lysogenic promoters paQ and pI [17], outside the switch region (Figure 1A). It is unclear whether the loss of plaque formation in response. We seek to characterise the proteolysis of 186-CII and its functional consequences in an effort to further understand the role of CII and its degradation in the 186 switch, and in switches with similar delayed negative feedback topologies
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