Functional analysis of heterologous holin proteins in a λΔS genetic background

Holins are small hydrophobic proteins causing non-specific membrane lesions at the end of bacteriophage multiplication, to promote access of the murein hydrolase to their substrate. We have established a λΔS genetic system, which enables functional expression of holins from various phages in an isogenic phage λ background, and allows qualitative evaluation of their ability to support lysis of Escherichia coli cells. Synthesis of Holins is under control of native λ transcription and translation initiation signals, and the temperature-sensitive CIts857 repressor. A number of different holins were tested in this study. The opposing action of phage λ S105 and S107 holin variants in lysis timing could be confirmed, whereas we found evidence for a functionally non-homologous dual translational start motif in the Listeria phage Hol500 holin, i.e., the Hol500-96 polypeptide starting at Met-1 revealed a more distinct lytic activity as compared to the shorter product Hol500-93. The largest holin known, HolTW from a Staphylococcus aureus phage, revealed an early lysis phenotype in the λΔSthf background, which conferred a plaque forming defect due to premature lysis. Mutant analysis revealed that an altered C-terminus and/or a V52L substitution were sufficient to delay lysis and enable plaque formation. These results suggest that the extensively charged HolTW C-terminus may be important in regulation of lysis timing. The gene 17.5 product of E. coli phage T7 was found to support sudden, saltatory cell lysis in the λΔSthf background, which clearly confirms its holin character. In conclusion, λΔSthf offers a useful genetic tool for studying the structure-function relationship of the extremely heterogeneous group of holin protein orthologs.