SummaryThe genomes of malaria parasites contain many genes of unknown function. To assist drug development through the identification of essential genes and pathways, we have measured competitive growth rates in mice of 2,578 barcoded Plasmodium berghei knockout mutants, representing >50% of the genome, and created a phenotype database. At a single stage of its complex life cycle, P. berghei requires two-thirds of genes for optimal growth, the highest proportion reported from any organism and a probable consequence of functional optimization necessitated by genomic reductions during the evolution of parasitism. In contrast, extreme functional redundancy has evolved among expanded gene families operating at the parasite-host interface. The level of genetic redundancy in a single-celled organism may thus reflect the degree of environmental variation it experiences. In the case of Plasmodium parasites, this helps rationalize both the relative successes of drugs and the greater difficulty of making an effective vaccine.

hrp genes, basic pathogenicity genes of the pepper and tomato pathogen Xanthomonas campestris pv. vesicatoria, are regulated dependent on environmental conditions. We isolated the hrpXv gene, which was found to be outside the large hrp cluster comprising the six loci hrpA to hrpF. The predicted HrpXv protein is 476 amino acids long and has a molecular mass of 52.5 kDa. HrpX is highly conserved among xanthomonads and is a member of the AraC family of regulatory proteins. An hrpXv insertion mutant has a typical hrp phenotype and no longer allows induction of the five hrp loci hrpB to hrpF in the new hrp induction medium XVM2, indicating that HrpXv is the positive regulator of these loci. An hrpXv mutant could be partially complemented by the related hrpB gene of Burkholderia solanacearum, the protein product of which shows 40 and 58% amino acid identity and similarity, respectively, to HrpXv. The hrpXv gene itself has a low basal level of expression that is enhanced in XVM2. Expression of hrpXv as well as that of the hrpA locus is independent of the hrpXv gene. The transcription start site of hrpXv was mapped. Comparison between the hrpXv promoter and the corresponding region of the hrpXc gene from X. campestris pv. campestris revealed sequence conservation up to position ؊84. A putative helix-turn-helix motif in the C-terminal region of HrpXv and its possible interaction with a conserved hrp promoter element, the plant-inducible promoter box, are discussed.The species Xanthomonas campestris has been described as being exclusively plant pathogenic and shows a high degree of host specificity. This finding has led to division of this species into many pathovars, according to the host ranges of strains. X. campestris pv. vesicatoria is the causal agent of bacterial spot disease of pepper and tomato plants and has served as a model system with which to study the molecular basis of pathogenesis. In a compatible interaction, the bacteria multiply in the intercellular spaces of the plant tissue, while in resistant plants, bacterial growth is inhibited and often accompanied by the hypersensitive reaction (HR), a rapid cell death (necrosis) at the site of infection (10, 11). The 23-kb chromosomal hrp (hypersensitive reaction and pathogenicity) gene cluster (6, 38) is indispensable for pathogenesis in susceptible host plants and for the elicitation of the HR in resistant host and nonhost plants (8). The cluster contains six hrp loci, hrpA to hrpF, which have been sequenced (7,21,22,54). Transposon mutagenesis with Tn3-gus that carries a promoterless ␤-glucuronidase (GUS) gene allowed expression studies of five out of six hrp loci (45). hrpA, hrpB, hrpC, hrpD, and hrpF are suppressed during growth in complex medium and are induced in planta and in TCM (tomato conditioned medium) (45). The hrp induction medium TCM, the quality of which varied, has recently been replaced by the synthetic medium XVM2, which has been shown to induce the hrpA locus (54). hrpE expression has not been studied yet because of lack of Tn3-gus insertio...

The malaria parasite suffers severe population losses as it passes through its mosquito vector. Contributing factors are the essential but highly constrained developmental transitions that the parasite undergoes in the mosquito midgut, combined with the invasion of the midgut epithelium by the malaria ookinete (recently described as a principal elicitor of the innate immune response in the Plasmodium-infected insect). Little is known about the molecular organization of these midgut-stage parasites and their critical interactions with the blood meal and the mosquito vector. Elucidation of these molecules and interactions will open up new avenues for chemotherapeutic and immunological attack of parasite development. Here, using the rodent malaria parasite Plasmodium berghei, we identify and characterize the first microneme protein of the ookinete: circumsporozoite- and TRAP-related protein (CTRP). We show that transgenic parasites in which the CTRP gene is disrupted form ookinetes that have reduced motility, fail to invade the midgut epithelium, do not trigger the mosquito immune response, and do not develop further into oocysts. Thus, CTRP is the first molecule shown to be essential for ookinete infectivity and, consequently, mosquito transmission of malaria.