Abstract
Synechococcus sp. PCC 7002 and many other cyanobacteria have two genes that encode key enzymes involved in chlorophyll a, biliverdin, and heme biosynthesis: acsFI/acsFII, ho1/ho2, and hemF/hemN. Under atmospheric O2 levels, AcsFI synthesizes 3,8-divinyl protochlorophyllide from Mg-protoporphyrin IX monomethyl ester, Ho1 oxidatively cleaves heme to form biliverdin, and HemF oxidizes coproporphyrinogen III to protoporphyrinogen IX. Under microoxic conditions, another set of genes directs the synthesis of alternative enzymes AcsFII, Ho2, and HemN. In Synechococcus sp. PCC 7002, open reading frame SynPCC7002_A1993 encodes a MarR family transcriptional regulator, which is located immediately upstream from the operon comprising acsFII, ho2, hemN, and desF (the latter encodes a putative fatty acid desaturase). Deletion and complementation analyses showed that this gene, denoted chlR, is a transcriptional activator that is essential for transcription of the acsFII-ho2-hemN-desF operon under microoxic conditions. Global transcriptome analyses showed that ChlR controls the expression of only these four genes. Co-expression of chlR with a yfp reporter gene under the control of the acsFII promoter from Synechocystis sp. PCC 6803 in Escherichia coli demonstrated that no other cyanobacterium-specific components are required for proper functioning of this regulatory circuit. A combination of analytical methods and Mössbauer and EPR spectroscopies showed that reconstituted, recombinant ChlR forms homodimers that harbor one oxygen-sensitive [4Fe-4S] cluster. We conclude that ChlR is a transcriptional activator that uses a [4Fe-4S] cluster to sense O2 levels and thereby control the expression of the acsFII-ho2-hemN-desF operon.
Highlights
A four-gene operon, acsFII-ho2-hemN-desF, is most highly expressed under microoxic conditions
Deletion of chlR Affects Growth at Lower O2 Levels—To investigate the function of open reading frames (ORFs) SynPCC7002_A1993, which was annotated as a putative transcriptional regulator, a deletion mutant was constructed by completely replacing the ORF by a drug resistance cassette (Fig. 1A)
We showed that the product of ORF SYNPCC7002_A1993, annotated as a putative transcriptional regulator, is an oxygen-sensitive transcriptional activator for an operon of four genes, acsFII, ho2, hemN, and desF
Summary
A four-gene operon, acsFII-ho2-hemN-desF, is most highly expressed under microoxic conditions. Under atmospheric O2 levels, AcsFI synthesizes 3,8-divinyl protochlorophyllide from Mg-protoporphyrin IX monomethyl ester, Ho1 oxidatively cleaves heme to form biliverdin, and HemF oxidizes coproporphyrinogen III to protoporphyrinogen IX Under microoxic conditions, another set of genes directs the synthesis of alternative enzymes AcsFII, Ho2, and HemN. AcsFI/ChlAI catalyzes the formation of the isocyclic ring of chlorophyll a (converting Mg-protoporphyrin IX monomethyl ester into 3,8-divinyl protochlorophyllide) under atmospheric oxygen levels, whereas under microoxic growth conditions, the same reaction is mainly performed by AcsFII/ChlAII [7]. Both enzymes are monooxygenases and require molecular O2 as a substrate. We show that this transcription activator harbors a single oxygen-sensitive [4Fe-4S] cluster per homodimer that acts as the O2 sensory prosthetic group
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
