How Do Microbial Pathogens Make CENs?

Abstract

In a variety of pathogens, drug resistance and aneuploidy areintimately associated. Anueploidy is believed to alter the dosage ofcertain genes that can impart drug resistance. Generation of a newchromosome by duplication of chromosome segments followed bytelomere addition in a pathogenic yeast Candida glabrata [1] andisochromosome formation by breakage of chromosome 5 at thecentromere followed by joining two identical arms of chromosome5 in an opportunistic yeast Candida albicans [2] have been shown tooccur frequently in drug-resistant isolates. Fluconazole-resistantstrains of another pathogenic fungus, Cryptococcus neoformans, havebeen shown to be disomic for certain chromosomes [3].Experimental evidence suggests that acquisition of chromosomesby the plant pathogenic fungus Fusarium can convert a non-pathogenic strain to a pathogenic one [4]. Anueploidy has beenshown to be the cause of drug resistance in the protozoan parasiteLeishmania as well [5].Improper chromosome segregation is one route to aneuploidy.The centromere–kinetochore complex facilitates interaction be-tween a chromosome and the spindle microtubules to ensure equalsegregation of chromosomes from a mother to daughter cells. Inaddition to serving as sites of protein assembly to formkinetochores, centromeres also hold two sister chromatids togetheruntil the onset of anaphase by balancing opposing forces acting onthem—a pole-ward (outward) force generated by spindle micro-tubules depolymerizing toward opposite poles, and a cohesive(inward) force between two sister chromatids. Paradoxically, inspite of performing the conserved function of chromosomesegregation, centromere (CEN) DNA sequence and organizationof CEN DNA elements vary widely among eukaryotes. In contrast,several kinetochore proteins are evolutionarily conserved, al-though sometimes this conservation is restricted to a group oforganisms. For example, the Dam1 complex, an outer kinetochoreprotein complex, is fungus specific and is essential for viability in C.albicans [6,7]. Thus, this complex may be a suitable target fordevelopment of anti-fungal drugs. However, a CEN-specifichistone H3 variant of the CENP-A/Cse4 family is found to beuniversally associated with the formation of specialized and uniquechromatin at all functional CENs despite seemingly diverse CENDNA sequences.