The unknown factor CSF could have been a non-protein factor (i.e, DNA) and lambda DNA would have been a good candidate for the same, since CII may be stabilized by binding to its cognate promoter. However, in our in vivo experiments, the plasmid pKP219 (used for the expression of exogenous CII) contained the promoter sequence PE, ruling out such a possibility. Stabilization of CII

in cells overexpressing hflKC is not surprising since HflKC is an inhibitor LY2874455 of CII-proteolysis. It is worthwhile to note that the effect of HflKC deletion is epistatic over the effect of cIII deletion, since even the absence of CIII cannot produce clear plaques in a ΔhflKC host. It is possible that CIII (and the hypothesized CIII-like factor CSF) works better in the absence of HflKC (Figure 5B). Therefore CII is better stabilized under these conditions GDC-0941 manufacturer and produces turbid plaques in ΔhflKC cells. cI, cII and cIII were first described as phage mutations which led to clear plaques in a wild type host. On the other hand, λ gives very turbid plaques in a ΔhflKC host. Our study thereby raises the possibility of finding novel phage mutations that would give clear plaques in an hflKC-deleted host.

Conclusions 1. E. coli HflKC inhibits the proteolysis of λCII by HflB and hence the overexpression of the former results in an increase in the lysogenic frequency. 2. In the absence of HflKC, λCII is stabilized upon infection by cIII-defective λ, suggesting

the presence of a yet unidentified phage factor CSF (CII-stabilizing factor). Acknowledgements We thank S. Adhya (NIH, Bethesda) for λcIII 67 and for his comments on the manuscript, Inositol oxygenase K. Ito and Y. Akiyama (Kyoto University) for E. coli AK990 strain, S. K. Dasgupta (Bose Institute) for the plasmid pSD5b and K. Shearwin (University of Adelaide) for anti-CII antibody. This work was funded by Institutional Project 5 (Microbial Genomics) of Bose Institute. KB was supported by CSIR, India (F. No. 9/15 (302)/2004-EMR-I). References 1. Avlund M, Dodd IB, Semsey S, Sneppen K, Krishna S: Why do phage play dice? J Virol 2009,83(22):11416–11420.4SC-202 PubMedCrossRef 2. Zeng L, Skinner SO, Zong C, Sippy J, Feiss M, Golding I: Decision making at a subcellular level determines the outcome of bacteriophage infection. Cell 2010,141(4):682–691.PubMedCrossRef 3. Court D, Green L, Echols H: Positive and negative regulation by the cII and cIII gene products of bacteriophage lambda. Virology 1975,63(2):484–491.PubMedCrossRef 4. Echols H, Green L: Establishment and maintenance of repression by bacteriophage lambda: the role of the cI, cII, and c3 proteins. Proc Natl Acad Sci USA 1971,68(9):2190–2194.PubMedCrossRef 5.