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bThe plastid of the malaria parasite, the apicoplast, is essential for parasite survival. It houses several pathways of bacterial origin that are considered attractive sites for drug intervention. Among these is the sulfur mobilization (SUF) pathway of Fe-S cluster biogenesis. Although the SUF pathway is essential for apicoplast maintenance and parasite survival, there has been limited biochemical investigation of its components and inhibitors of Plasmodium SUFs have not been identified. We report the characterization of two proteins, Plasmodium falciparum SufS (PfSufS) and PfSufE, that mobilize sulfur in the first step of Fe-S cluster assembly and confirm their exclusive localization to the apicoplast. The cysteine desulfurase activity of PfSufS is greatly enhanced by PfSufE, and the PfSufS-PfSufE complex is detected in vivo. Structural modeling of the complex reveals proximal positioning of conserved cysteine residues of the two proteins that would allow sulfide transfer from the PLP (pyridoxal phosphate) cofactor-bound active site of PfSufS. Sulfide release from the L-cysteine substrate catalyzed by PfSufS is inhibited by the PLP inhibitor D-cycloserine, which forms an adduct with PfSufS-bound PLP. D-Cycloserine is also inimical to parasite growth, with a 50% inhibitory concentration close to that reported for Mycobacterium tuberculosis, against which the drug is in clinical use. Our results establish the function of two proteins that mediate sulfur mobilization, the first step in the apicoplast SUF pathway, and provide a rationale for drug design based on inactivation of the PLP cofactor of PfSufS.

SummaryThe apicoplast of Plasmodium falciparum carries a 35 kb circular genome (plDNA) that replicates at the late trophozoite stage of the parasite intraerythocytic cycle. plDNA replication proceeds predominantly via a D-loop/bi-directional ori mechanism with replication ori localized within inverted repeat region. Although replication of the apicoplast genome is a validated drug target, the proteins involved in the replication process are only partially characterized. We analysed DNA-protein interactions at a plDNA replication ori region and report the identification of a nuclearencoded DnaJ homologue that binds directly to ori elements of the plDNA molecule. PfDnaJA interacted with the minor groove of the DNA double-helix and recognized a 13 bp sequence within the ori. Inhibition of binding with anti-PfDnaJA antibodies confirmed identity of the protein in DNA-binding experiments with organellar protein fractions. The DNA-binding domain of the~69 kDa PfDnaJA lay within the N-terminal 38 kDa region that carries DnaJ signature motifs. In contrast to PfDnaJA in parasite organellar fractions, the recombinant protein interacted with DNA in a sequence non-specific manner. Our results suggest a role for PfDnaJA in replication/repair of the apicoplast genome.

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An innovative and efficient approach towards diversity‐oriented synthesis of 4‐phenacylideneflavenes has been developed from substituted salicylaldehydes and acetophenones using iodine under solvent‐free conditions. Both symmetrical and unsymmetrical functionalized 4‐phenacylideneflavenes were synthesized in good to excellent yields and their mechanism of formation is discussed.