The chlorarachniophyte nucleomorph is supplemented with host cell nucleus-encoded histones

In chlorarachniophytes, algae originating from secondary endosymbiosis, the complex plastids retained a nucleomorph, the vestigial nucleus of the green algal endosymbiont. The nucleomorph of Bigelowiella natans encodes several plastid-targeted proteins and hundreds of housekeeping proteins. However, many fundamental genes for the maintainance of this subcompartment are missing. In this issue of Molecular Microbiology, Hirakawa et al. (2011) demonstrate nuclear histone genes of dual evolutionary origin in B. natans and convincingly show the targeting of the corresponding proteins to nucleus and nucleomorph respectively. One of the ways through which the nuclear genome exerts control upon its endosymbiotic junior partner is revealed. Insights into the nature of bipartite targeting sequences directing the respective proteins into the periplastidal space (where the nucleomorph resides) are gained. Further, cell cycle-dependent, differential regulation is shown for both nuclear and nucleomorph histone genes.