Overlapping transcriptional units in the deo operon of Escherichia coli K-12 : Evidence from phage Mu-1 insertion mutants

Abstract

Four closely-linked genes of Escherichia coli, comprising the deoxyribonucleoside ( deo) operon, viz. dra, tpp, drm, and pup † † Abbreviations used: dra, gene coding for deoxyriboaldolase; tpp, thymidine phosphorylase; drm, deoxyribomutase; pup, purine nucleoside phosphorylase; cdd, cytidine deaminase; udp, uridine phosphorylase; upp, uridine monophosphate pyrophosphorylase. cytR and deoR: regulatory genes for the deo operon. The symbol:: indicates that the Mu prophage given in parentheses is inserted in the gene preceding the symbol. FUrdMP, 5-fluorouridine monophosphate; FdUrdMP, 5-fluorodeoxyuridine monophosphate; FUra, 5-fluorouracil; FdUrd, 5-fluorodeoxyuridine; PRPP, 5-phosphoribosyl-1-pyrophosphate; dA, deoxyadenosine; dR-1-P, deoxyribose-1-phosphate; dR-5-P, deoxyribose-5-phosphate; Ade. adenine. CRP, cyclic AMP receptor protein; cAMP, cyclic adenosine 3′:5′-monophosphate, O, operator; P, promoter. , which code for enzymes involved in the catabolism of nucleosides and deoxynucleosides, are known to form two transcriptional units, one initiated from a promoter to the left of dra and one initiated between tpp and drm. Thus a potentially polar mutation induced by phage Mu-1 in tpp has no effect on the level of drm gene product. We show here, however, that if the level of transcription of all four genes is increased by the insertion of a regulatory mutation ( deoR − or cytR −) then a Mu-induced polar mutation in tpp does in fact drastically reduce the level of drm gene product. The interpretation of this result is that two species of mRNA are produced; one is tetracistronic, being transcribed from a promoter located to the left of dra, and one is dicistronic, being transcribed from a promoter lying between tpp and drm. The two distal genes are therefore transcribed into two separate mRNA species. In the wild-type ( deoR + cytR +) most of the drm and pup gene products are synthesized from the short message and thus there is no polar effect of a Mu-1-induced tpp mutation on drm. The long message is controlled by the deoR and cytR gene products, whereas the remaining inducibility of drm in deoR − cytR − strains with a polar tpp mutation suggests that the short message is controlled by another, unknown, regulatory system.