Instability of CII is needed for efficient switching between lytic and lysogenic development in bacteriophage 186

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.

Analysis of Infection Time Courses Shows CII Levels Determine the Frequency of Lysogeny in Phage 186

“…We hypothesised that, like λ, the cellular concentration of 186 CII is the critical factor in its lytic → lysogenic decision. Consistent with this, a 186 phage expressing a stabilised version of CII has a frequency of lysogeny (FOL) close to 100% [17].…”

“…Using this model and the trimmed data, the FOL, f, for phage 186 + was estimated to be ~18%. This value is slightly higher than the ~10% previously obtained from single round phage plating experiments performed at low MOAs [17]. However, it should be emphasised that the infection time course experiment involves multiple rounds of infection, and thus f reflects an aggregate lysogenisation frequency from multiple rounds of infection under changing conditions.…”

“…Extremes of CII activity are known to have strong effects on the frequency of lysogeny in phage 186. In the absence of active CII, 186 establishes lysogeny only rarely [17,21]. In contrast, high level CII expression due to a stabilising mutation in cII results in ~100% lysogeny [17].…”

“…Phage 186 expresses a protein, also called CII, which is essential for establishment of lysogeny and regulates gene expression in a similar manner to the unrelated λ CII protein. 186 CII is produced from the early lytic pR transcript and activates transcription of the lysogenic operon from the pE promoter to express the CI lysogenic repressor and the 186 integrase [17,18] (Figure 1). 186 CI enforces lysogeny by repressing the early lytic promoters, including pR, and activating its own promoter, pL [15].…”

“…CI also represses the promoter for the late gene activator B [19]. 186 CII is rapidly degraded by the host RseP and FtsH proteases in vivo [17]. We hypothesised that, like λ, the cellular concentration of 186 CII is the critical factor in its lytic → lysogenic decision.…”