Replication of plasmids derived from Shiga toxin-converting bacteriophages in starved Escherichia coli

Abstract

The pathogenicity of Shiga toxin-producingEscherichia coli(STEC) depends on the expression ofstxgenes that are located on lambdoid prophages. Effective toxin production occurs only after prophage induction, and one may presume that replication of the phage genome is important for an increase in the dosage ofstxgenes, positively influencing their expression. We investigated the replication of plasmids derived from Shiga toxin (Stx)-converting bacteriophages in starvedE. colicells, as starvation conditions may be common in the intestine of infected humans. We found that, unlike plasmids derived from bacteriophageλ, the Shiga toxin phage-derived replicons did not replicate in amino acid-starvedrelA+andrelA−cells (showing the stringent and relaxed responses to starvation, respectively). The presence of the stable fraction of the replication initiator O protein was detected in all tested replicons. However, while ppGpp, the stringent response effector, inhibited the activities of theλPRpromoter and its homologues from Shiga toxin-converting bacteriophages, these promoters, except forλPR, were only weakly stimulated by the DksA protein. We suggest that this less efficient (relative toλ) positive regulation of transcription responsible for transcriptional activation of theorigincontributes to the inhibition of DNA replication initiation of Shiga toxin-converting bacteriophages in starved host cells, even in the absence of ppGpp (as in starvedrelA−hosts). Possible clinical implications of these results are discussed.