The (p)ppGpp levels of the host seem to act as a sensor for phage lambda development, primarily affecting transcription. Modest ppGpp levels inhibit pR and active pE, pI, and paQ promoters in vivo and have effects in vitro that seem to favor lysogeny. In contrast, absent or high concentrations of (p)ppGpp favor lysis. Modest ppGpp levels favor lysogeny by leading to low HflB (FtsH). When ppGpp is either absent or high, HflB protease levels are high; this leads to lower CII (a lysogeny-promoting phage protein)
and favors lysis.