Immediately after DNA entry, lytic phages defeat bacterial defenses and hijack host cell machineries to establish a favourable environment for their multiplication. The early-expressed genes governing host takeover are highly diverse from one phage to another and most of them have no assigned function. In spite of their potential to control, subvert or disrupt bacterial cell vital functions, their characterization is still at an early stage.
Bacteriophage T5 uses a unique two-step mechanism to deliver its 121 kb DNA into its host Escherichia coli. At the onset of infection only 8% of the genome enters the cell before the transfer temporarily stops. During the pause, expression of the genes encoded by this short DNA fragment has immediate devastating effects on the host: the bacterial chromosome is degraded, the expression of the host genes is rapidly shut off and the defense restriction/methylation and RecBCD DNA repair systems are inactivated. After a few minutes, T5 DNA transfer resumes allowing entry of the complete genome. Expression of these pre-early genes is then rapidly shut-off and expression of the middle and late genes of T5 is activated, initiating phage productive growth. Two of the pre-early genes, A1 and A2 are required for resuming and completing phage DNA transfer. Although these events were described 50 years ago, very little is known about the molecular and cellular mechanisms of how T5 regulates the two-step DNA transfer and takes control of its host. We have overexpressed A1 and A2 in E. coli cells and will present here the first functional characterization of the purified proteins. Their role in the early steps of T5 infection will be discussed.