Expression of λ red genes restores recB-dependent protein X induction in E. coli

Abstract

DNA DAMAGE and/or inhibition of DNA synthesis elicit a number of responses in Escherichia coli known as SOS functions¹, including the production of large quantities of a protein provisionally called protein X (ref. 2). Induction of protein X, like the other SOS functions, requires functional rec A and lex A genes^3,4. Gudas and Pardee⁴ proposed a model to suggest roles for rec A and lex A in the regulation of protein X production. Several research groups have identified protein X as the rec A gene product^5–8 which prompted them to formulate updated models, to account for the apparent autoregulation of the rec A gene^6–8. These models propose that the lex A gene codes for a represser which inhibits the transcription of the rec A gene. Following treatment, such as exposure to ultraviolet irradiation or nalidixic acid, a small inducer molecule is produced that causes the rec A gene product, present in low levels in untreated cells, to inactivate the lex A repressor. This allows transcription of the rec A gene that results in the production of large amounts of the rec A gene product, protein X. Gudas and Pardee⁴ suggested that the inducer molecule may be a DNA degradation product resulting from the action of the recBC gene products, exonuclease V. In support of this idea they showed that protein X is not induced by nalidixic acid in recB⁻ mutants deficient in exonuclease V. On the other hand, Little and Hanawalt⁹ showed that intracellular degradation of unmodified phage DNA by restriction is not sufficient to induce protein X, and they have challenged the role of DNA degradation in the induction process. In this report we show that phage λ genes controlling λ exonuclease can restore the missing function for the induction of protein X in recB⁻ mutants. This is further evidence that DNA degradation is required to derepress the rec A gene.