In bacteria, cytoplasmic levels of the effector
nucleotide ppGpp are regulated in response to changes in growth conditions. This study describes the involvement of SpoT-mediated
ppGpp accumulation in the survival of light-exposed bacteria during
fatty acid starvation. In contrast to isogenic wild-type strains and relA mutants, the 'Vibrio angustum' S14 spoT and Escherichia coli relA spoT mutants displayed significant losses in viability in response to
cerulenin-induced
fatty acid starvation under cool-white fluorescent light. However, when
starvation experiments were performed in complete darkness, or under light filtered through a UV-resistant perspex sheet, only a minor decline in viability was observed for the wild-type and mutant strains. This finding indicated that the lethal effect was mediated by weak UV emission. In contrast to the E. coli relA spoT mutant, which lacks
ppGpp, the 'V. angustum' S14 spoT mutant exhibited higher
ppGpp levels and lower
RNA synthesis rates during
fatty acid starvation, features that might be correlated with its lethality. In agreement with this finding,
fatty acid starvation lethality also occurred upon induction of
ppGpp overaccumulation in E. coli. These data suggest that the precise regulation of
ppGpp levels in the stressed cell is crucial, and that both the absence and the overaccumulation of
ppGpp impair
fatty acid starvation survival of light-exposed cells. Moreover, the UV-induced lethal effect during
fatty acid starvation was also observed for E. coli strains mutated in rpoS and dps, which, in the wild-type, are regulated directly or indirectly by
ppGpp, respectively. The restoration of viability of
fatty-acid-starved spoT mutant cells through the addition of exogenous
catalase suggested that the observed light-dependent lethal effect was, at least in part, caused by UV-imposed oxidative stress. Based on these results, it is proposed that
fatty acid starvation adaptation of light-exposed bacterial cells depends on the development of resistance to UV-induced oxidative stress. This stress resistance was found to require appropriate
ppGpp levels,
ppGpp-induced RpoS expression and, hence, upregulation of RpoS-regulated stress-defending genes, such as dps.