Effect of endogenous carotenoids on "adaptive" mutation in Escherichia coli FC40

The appearance over many days of Lac(+) frameshift mutations in Escherichia coli strain FC40 incubated on lactose selection plates is a classic exampl e of apparent "adaptive" mutation in an episomal gene. We show that endogen ously overproduced carotenoids reduce adaptive mutation under selective con ditions by a factor of around two. Carotenoids are known to scavenge single t oxygen suggesting that the accumulation of oxidative base damage may be a n integral part of the adaptive mutation phenomenon. If so, the lesion cann ot be 7,8-dihydro-8-oxoguanine since adaptive mutation in FC40 is unaffecte d by mutM and mutY mutations. If active oxygen species such as singlet oxyg en are involved in adaptive mutation then they should also induce frameshif t mutations in FC40 under non-selective conditions. We show that such mutat ions can be induced under non-selective conditions by protoporphyrin photos ensitisation and that this photodynamic induction is reduced by a factor of just over two when endogenous carotenoids are present. We argue that the i nvolvement of oxidative damage would in no way be inconsistent with current understanding of the mechanism of adaptive mutation and the role of DNA po lymerases. (C) 2001 Elsevier Science B.V. All rights reserved.