The research, led by Yasmina Drissi-El Boukili and Anna Rosanas-Urgell together with colleagues at the Clinical Research Unit of Nanoro and the Institute of Tropical Medicine in Antwerp, examined people naturally infected with Plasmodium falciparum, the deadliest of the malaria parasites that infect humans. The team focused on mutations in the beta-globin gene, HBB, which encodes one of the protein chains of hemoglobin, the oxygen-carrying molecule inside red blood cells. Two variant hemoglobins in particular drew their attention: hemoglobin S and hemoglobin C, both of which are common in West Africa precisely because they confer survival advantages against falciparum malaria.

People who carry one normal copy of the gene and one variant copy produce red blood cells containing a mixture of hemoglobins, a condition known as the HbAS trait for hemoglobin S or HbAC for hemoglobin C. These heterozygous states are famously protective: carriers are far less likely to develop the life-threatening complications of malaria, which is why natural selection has maintained these variants at high frequency in malaria-endemic regions. What has remained far less clear is whether these altered red blood cells affect another dimension of the parasite’s life cycle, namely its ability to commit to producing the sexual stages that make transmission possible.

That commitment process, called sexual conversion, is the opening move of gametocytogenesis. Most parasites circulating in a patient’s blood are asexual forms that multiply relentlessly inside red blood cells, consuming hemoglobin and driving the fevers and anemia of clinical malaria. But a small fraction of parasites instead differentiates into gametocytes, the sexual forms that lie dormant in the bloodstream until a mosquito takes a blood meal and ingests them. Inside the mosquito, these gametocytes transform again into gametes, fertilize, and give rise to the sporozoites that the insect will inject into its next human victim. Without sexual conversion, the entire transmission chain would collapse, which makes the factors that regulate this switch a matter of intense interest for anyone hoping to block the spread of malaria.

Measuring sexual conversion in natural infections, however, has long been technically difficult. To overcome this obstacle, the researchers developed a new ex vivo assay, which they called the evSCA, designed to quantify how frequently parasites drawn directly from infected patients commit to the sexual pathway. Using blood samples collected in Nanoro, a malaria-endemic district in central Burkina Faso, they compared sexual conversion rates among infected individuals grouped by their HBB genotype: those with the standard HbAA hemoglobin, those with the sickle-cell trait HbAS, and those with the HbAC variant.

The results were striking. Sexual conversion rates were significantly higher in infected individuals carrying HbAS or HbAC than in those with wild-type HbAA red blood cells. In other words, the very red blood cells that shield their owners from severe disease appeared to be nudging the parasites inside them toward the sexual pathway at a greater rate. Because gametocytes are the only forms the mosquito can pick up, elevated sexual conversion could in principle raise the number of transmissible parasites in the carrier’s blood, with consequences for how malaria spreads through communities where these hemoglobin variants are common.

To test whether this association reflected a direct effect of the red blood cell environment rather than some other difference between patients, the team turned to a controlled in vitro system. They cultured parasites in red blood cells of different hemoglobin compositions and measured sexual conversion using a reporter parasite line, NF54-gexp02-Tom, in which the expression of a fluorescent marker is tied to the activation of ap2-g, the master regulator gene that triggers sexual commitment. This reporter line allowed the researchers to count precisely how many parasites flipped the genetic switch under each culture condition. Consistent with the ex vivo findings from patient samples, parasites grown in HbAS red blood cells showed higher sexual conversion rates than parasites grown in HbAA cells, supporting the idea that the altered intracellular environment of variant-hemoglobin red blood cells actively promotes the sexual switch.

The researchers also considered an alternative explanation rooted in immunity rather than in the red blood cell itself. Antibodies directed against infected red blood cells can shape which parasite stages survive and flourish in a given host, so differences in antibody responses between HBB genotypes might conceivably account for the observed pattern. To investigate this, the team used flow cytometry to quantify immunoglobulin G and immunoglobulin M responses against antigens on the surface of red blood cells infected with trophozoites, the actively feeding asexual stage, and with stage I gametocytes, the earliest sexual stage. When they compared plasma from individuals with different HBB genotypes, they found no significant differences in these antibody responses, arguing against humoral immunity as the driver of the elevated sexual conversion and leaving the red blood cell environment itself as the more likely culprit.

The precise molecular mechanism by which variant hemoglobins encourage sexual commitment remains to be worked out, but the finding adds a fascinating new layer to the long evolutionary story of hemoglobin variants and malaria. Hemoglobin S and hemoglobin C are classic examples of a genetic trade-off: they cost carriers some fitness burden, including, in the case of hemoglobin S, the risk of sickle-cell disease when inherited from both parents, in exchange for protection against a killer disease. The new data suggest the ledger may be even more complicated than previously thought. If carriers of these variants, while suffering less severe illness, also harbor parasites that are more prone to becoming transmissible gametocytes, then human genetics could be quietly shaping the transmission landscape of malaria in endemic regions, influencing how intensively the parasite circulates even as it causes less disease in protected individuals.

From a public health perspective, the study opens several avenues worth pursuing. Understanding how the red blood cell environment modulates ap2-g activation could reveal new targets for transmission-blocking interventions, an area of growing importance as malaria control programs aim not merely to treat sick patients but to interrupt the parasite’s passage from human to mosquito to human. The ex vivo assay developed by the team also provides a practical tool for measuring sexual conversion directly in natural infections, something that has been difficult to achieve and that could now be applied to larger and more diverse populations. As researchers dig deeper into how host genetics sculpt the parasite’s life cycle decisions, the humble red blood cell is emerging not just as the parasite’s home and food source, but as an active participant in determining whether an infection remains a private battle between host and pathogen or becomes the next link in a chain of transmission.

Subject of Research: Effect of HBB hemoglobin variants on Plasmodium falciparum sexual conversion in naturally infected malaria patients

Article Title: Elevated Plasmodium falciparum sexual conversion in HbAC and HbAS red blood cells in naturally infected malaria patients

Article References: Elevated Plasmodium falciparum sexual conversion in HbAC and HbAS red blood cells in naturally infected malaria patients. (n.d.). https://doi.org/10.1371/journal.ppat.1014542

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014542

Keywords: malaria, Plasmodium falciparum, gametocytes, sexual conversion, hemoglobin S, hemoglobin C, HBB gene, red blood cells, Burkina Faso, transmission, ap2-g, ex vivo assay

The research team began with a systematic question about a large and underexplored molecular family. Ubiquitination, the process by which the small protein ubiquitin is attached to target proteins, governs countless cellular activities in eukaryotes, from protein degradation to DNA repair and cell differentiation. The process requires a cascade of enzymes: E1 activating enzymes priming ubiquitin, E2 conjugating enzymes transferring it, and E3 ligases conferring substrate specificity. While ubiquitination was known to operate in Plasmodium parasites, whether any particular ubiquitination machinery specifically controlled the sexual conversion step was unknown. To find out, the researchers deployed CRISPR–Cas9 mutagenesis screens against the complete repertoire of E2 ubiquitin-conjugating enzyme genes in the rodent malaria parasite Plasmodium yoelii, systematically deleting each of the fourteen E2 genes and assessing the consequences for parasite development across the life cycle.

The screen produced a clear winner. An E2 enzyme the authors designate Rad6B emerged as essential for gametocytogenesis, the process by which asexual blood-stage parasites convert into gametocytes. When Rad6B was deleted, gametocyte production collapsed to severe defects, and the mutant parasites proved dramatically impaired in their ability to form oocysts in the mosquito midgut, the critical developmental stage that follows ingestion of gametocytes during a blood meal. Parasitemia and host survival in mice infected with the Rad6B deletion mutant remained comparable to wild-type infections, indicating that the enzyme’s essential function was not in general blood-stage proliferation but specifically in the developmental switch that enables transmission. Whole genome sequencing and Sanger sequencing confirmed that the observed phenotypes stemmed cleanly from the targeted gene disruptions rather than off-target alterations.

Rad6B did not act alone. Through interaction analyses, the team identified its partner: a RING-type E3 ubiquitin ligase called Tex1. The pairing follows a well-established logic in ubiquitin biology, in which the E2 enzyme carries activated ubiquitin while the RING-domain E3 ligase recognizes the substrate and catalyzes the transfer. Deleting Tex1 phenocopied the loss of Rad6B, producing severe defects in gametocyte production and oocyst formation, and the two proteins were shown to function as a complex. To verify that the enzymatic activities of both partners were required, the researchers engineered point-mutant parasites: a Rad6B variant in which the conserved catalytic cysteine in the ubiquitin-conjugating domain was substituted, and Tex1 variants in which conserved arginine and lysine residues of the RING domain were mutated or the domain deleted outright. In every case, the disruption of catalytic function abolished the pathway’s effect on gametocytogenesis, confirming that genuine ubiquitin transfer—not a scaffolding role—lay behind the phenotype.

The substrate that the Rad6B–Tex1 complex targets turned out to be histone H2B, one of the core proteins around which DNA is wound. The complex catalyzes monoubiquitination of H2B, the attachment of a single ubiquitin molecule to a specific lysine residue on the histone. This modification has an illustrious history in biology: it was first characterized in yeast, where the Rad6 ortholog and its RING partner Bre1 mark H2B, and it was subsequently shown to regulate gene silencing, H3 methylation and differentiation of multipotent stem cells in mammals. The new study demonstrates that the same modification operates in Plasmodium yoelii and, crucially, in Plasmodium falciparum, the parasite responsible for the most deadly form of human malaria. Using ChIP-seq and CUT&Tag-seq, the researchers mapped H2B monoubiquitination across the parasite genome and found it dramatically reduced at specific loci in both the Rad6B and Tex1 deletion mutants.

The mechanistic chain then became clear through a series of elegant chromatin analyses. In wild-type parasites, H2B monoubiquitination at the ap2-g promoter region promotes the occupancy of tri-methylated histone H3 lysine 4, or H3K4me3, an activating histone mark classically associated with open, transcriptionally permissive chromatin. When the Rad6B–Tex1 complex was absent, this H3K4me3 enrichment at the ap2-g promoter was lost, ap2-g expression failed to rise, and the parasites remained locked in the asexual program. CUT&Tag-seq in the deletion mutants showed parallel losses of both H2Bub and H3K4me3 signals at the ap2-g locus, with other chromatin marks such as H3K79me3 and heterochromatin protein 1 distributions assessed for comparison. The effect was spatially specific: other AP2 family transcription factors and regulatory genes, including ap2-g2, ap2-g3, ap2-g5, hda2 and hp1, showed no comparable loss of H2Bub, indicating that the pathway does not broadly remodel the chromatin landscape but instead focuses on the master regulator of sexual commitment.

To prove that the histone mark itself, rather than some other function of Rad6B, was the causal agent, the team performed knock-in experiments that border on molecular surgery. They generated parasite lines in which the endogenous histone H2B was replaced with versions carrying substitutions at the ubiquitination site, and similarly modified H3 at the lysines subject to methylation. Parasites carrying an H2B version that cannot be monoubiquitinated failed to activate ap2-g and could not commit to gametocytogenesis, while restoring the ubiquitin-acceptable residue restored the pathway. Complementary experiments showed that altering the H3 lysine targeted for tri-methylation likewise disrupted sexual commitment, placing H2B monoubiquitination upstream of H3K4me3 in a coherent signaling relay. Transcriptomic comparisons reinforced the picture: genes downregulated in the Rad6B and Tex1 mutants overlapped substantially, and their orthologs in P. falciparum and P. berghei were enriched for genes whose expression peaks during gametocyte development, indicating that the two enzymes converge on the sexual differentiation program.

Perhaps the most consequential finding is the conservation of this mechanism in the human parasite. In Plasmodium falciparum, disruption of either Rad6B or Tex1 produced the same chromatin consequences—loss of H2Bub and H3K4me3 at the ap2-g locus—and the same failure to produce gametocytes, while asexual blood-stage growth of the mutants proceeded normally across three erythrocytic cycles. Using a parasite line in which ap2-g expression is tagged with green fluorescent protein, the researchers sorted individual trophozoites that had committed to sexual development from those that had not. The committed, AP2-G-positive parasites displayed significantly enriched H2Bub and H3K4me3 at the ap2-g upstream region compared with their AP2-G-negative siblings. The team also manipulated culture conditions known to influence sexual conversion, growing parasites under commitment-inducing conditions of high parasitemia with reduced albumin supplementation, and observed that these inducing conditions were accompanied by increased H2Bub enrichment at the ap2-g promoter, linking the epigenetic mark to the environmental cues that naturally trigger gametocyte production.

The implications for malaria control are considerable. Transmission-blocking interventions—drugs, vaccines or other strategies that prevent parasites from completing their development in the mosquito—remain a critical but underserved frontier in malaria elimination, because a treatment that kills asexual parasites while leaving gametocyte production intact can still leave the treated patient infectious to mosquitoes. By identifying the Rad6B–Tex1 ubiquitin ligase complex as the enzymatic gatekeeper of sexual commitment, the study supplies a pair of concrete molecular targets whose inhibition would suppress gametocyte formation at its epigenetic source. Because the pathway is conserved between rodent and human malaria parasites, the P. yoelii system can serve as a tractable model for screening inhibitors against the human equivalents. Beyond the translational angle, the work adds malaria parasites to the growing list of eukaryotes in which the ancient Rad6–Bre1 module of H2B monoubiquitination, first described in yeast more than two decades ago, serves as a master regulator of cell differentiation—here repurposed by a single-celled pathogen to choreograph one of the most consequential decisions in its life cycle, and in doing so, to choreograph the spread of a disease that still claims hundreds of thousands of lives each year.

Subject of Research: Epigenetic control of sexual commitment in malaria parasites via histone H2B monoubiquitination

Article Title: Histone H2B monoubiquitination drives sexual commitment in malaria parasites

Article References: Jiao, Z., Lin, L., Tang, R., Zhong, C.-Q., Wu, C., Li, W., Xu, R., Gao, Y., He, P., Wang, C., Zhang, Q.-Q., Liu, S., Yuan, J., Zhang, Q., & Li, J. (2026). Histone H2B monoubiquitination drives sexual commitment in malaria parasites. Nature Microbiology. https://doi.org/10.1038/s41564-026-02475-4

Image Credits: AI Generated

DOI: 10.1038/s41564-026-02475-4

Keywords: malaria, Plasmodium, gametocytogenesis, histone H2B monoubiquitination, Rad6B, Tex1, AP2-G, epigenetics, ubiquitin ligase, H3K4me3, sexual commitment, transmission blocking