Mutant/mutation
The mutant lacks expression of MRE11 and expresses GFP under the constitutive eef1a promoter.

Protein (function)
Double-strand breaks in the DNA resulting from innate DNA replication errors or exposure to radiation and chemical mutagens activate the DSB repair (DSBR) pathway, facilitating repair via two distinct mechanisms: “error-prone” non-homologous end-joining (NHEJ) and “error-free” homologous recombination (HR). In Plasmodium, HR appears to be the predominant DDR pathway, with homologues of many key players of mammalian HR. A key initiator of the HR pathway is meiotic recombination 11 protein (MRE11), which in mammals is part of the MRN/X complex that includes MRE11, RAD50 and NBS1 (Nijmegen breakage syndrome 1, a homologue of Saccharomyces cerevisae Xrs2). This complex has a crucial role in DDR during mitosis, promoting HR between sister chromatids to repair mutations arising during DNA replication. MRE11 is not essential in Trypanosoma brucei bloodstream forms, but its deletion results in impaired HR, reduced growth and increased sensitivity to DNA double-strand breaks. Numerous proteins potentially involved in HR have been identified in P. falciparum, although NBS1 is not encoded in the genome. In a recent study, a single mre11 orthologue containing 2 incomplete but catalytically active MPP_MRE11 domains was identified in P. falciparum (Gene ID: PF3D7_0107800), and complementation experiments confirmed DDR activity in response to DNA damage. A role in DDR was confirmed when P. falciparum mre11 (along with its MRN partner rad50) was shown to be significantly upregulated in response to treatment to induce DNA damage with the alkylating agent, methyl methanesulfonate (MMS).

Phenotype
There were significantly fewer oocysts in Δmre11 lines, and the vast majority of these oocysts were significantly smaller than WT-GFP oocysts. In none of the Δmre11 oocysts was there any evidence of sporozoite development, and those of similar size to WT-GFP oocysts had patterns of fragmented GFP expression and reduced Hoechst DNA-staining. Investigation of the sex-cell lineage of this defect revealed that it could be rescued by crossing Δmre11 parasites with Δmap2 (male-defective) cells, suggesting that the function of MRE11 is inherited through the female. The Δmre11 oocysts had a fully formed wall but the cytoplasm exhibited evidence of degeneration, with vacuolization and organelle breakdown, while the nuclei showed marked swelling of the nuclear membranes 

Additional information
Evidence is presented that: 
Absence of PbMRE11 results in significant transcriptional downregulation in activated gametocytes of genes involved in ribonucleoprotein biogenesis, spliceosome function and iron-sulphur cluster assembly. 

MRE11 is known to regulate checkpoint signaling during meiosis in other eukaryotes, and since a normal (4N) DNA content was observed in mature ookinetes it is possible that it functions in the reductive division of meiosis in early oocyst development. A previous study analyzing the Plasmodium meiotic recombinase Disrupted Meiotic cDNA 1 (DMC1) showed deletion of this genes resulted in a similar phenotype to Δmre11 mutants, with significant reduction (up to 80%) in oocyst numbers, which were smaller compared to WT lines and transmission was completely ablated. However, unlike Δmre11 lines sporogony did occur but was slower and limited numbers of nuclei were observed; whereas in Δmre11 lines sporogony was completely ablated.

Mre11 mRNA is present in cells throughout the P. berghei life-cycle (except merozoites), based on single cell RNA-seq data, with the highest abundance in ookinetes/oocysts.

Analyses of a mutant expressing a C-terminal GFP-tagged version of MRE11 (RMgm-4877) showed the following:
MRE11-GFP was not present in asexual blood stage parasites, but was detected in the first sexual stages, more specifically in the nucleus of female but not male gametocytes. Following gametocyte activation, female, but not male, gametocytes continued to express the protein. After fertilization the protein remained associated with the nucleus throughout zygote/ookinete development; whereas in oocysts the expression was diffuse and in sporozoites it was focused at a single point adjacent to the nuclear DNA  

Other mutants