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Nucleus- and nucleomorph-targeted histone proteins in a chlorarachniophyte alga.
Mol Microbiol. 2011 Jun; 80(6):1439-49
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29 Jun 2011
New Finding
DOI: 10.3410/f.11626956.12689055
Chlorarachniophyte algae are evolutionarily important because their secondary plastids are associated with a nuclear remnant from the green algal endosymbiont. Essentially nothing is known about how this remnant -- the nucleomorph -- is controlled. This paper shows that in Bigelowiella natans, two histones are imported from the cytoplasm to the nucleomorph, and their expression is coordinated with that of the 'host cell' nuclear histones. The authors show that the two histones concerned have undergone such rapid sequence changes relative to the host nuclear histones that phylogenetic assignment is not really possible.
In order to synchronise the cells, the authors developed a light/dark cycle protocol, revealing, on the way, that S phase and cell division preferentially occur in the dark. The symbiont histone mRNAs actually appeared slightly before the host histone mRNAs. This provokes a comparison with other organisms -- for example, in African trypanosomes, the mitochondrial DNA assemblage (kinetoplast) reproducibly divides before nuclear division occurs {1}. The Bigelowiella nucleomorph and nuclear histone genes all share a common potential regulatory element in the presumed promoter region, but additional transcriptional or post-transcriptional controls must be present since the regulation of symbiont H2B, in particular, is much less marked than that of the host H2B. (This strong regulation is actually paradoxical, given that the authors found relatively poor synchronisation of the cells.)
The paper raises many questions-- including how the synchronisation works and the mechanism of regulation. What a pity so few people work on this organism.
References
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The developmental cell biology of Trypanosoma brucei.
J Cell Sci 2005 Jan 15; Pt 2(118):283-90
PMID: 15654017
Disclosures
None declared
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Clayton C: F1000Prime Recommendation of [Hirakawa Y et al., Mol Microbiol 2011, 80(6):1439-49]. In F1000Prime, 29 Jun 2011; DOI: 10.3410/f.11626956.12689055. F1000Prime.com/11626956#eval12689055
F1000Prime Recommendations, Dissents and Comments for [Hirakawa Y et al., Mol Microbiol 2011, 80(6):1439-49]. In F1000Prime, 19 Jun 2013; F1000Prime.com/11626956
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The plastid of chlorarachniophytes is distinguished by the retention of a relict nucleus (nucleomorph) derived from a green algal endosymbiont, which is located in the periplastidal compartment (PPC). The nucleomorph genome of a chlorarachniophyte, Bigelowiella natans, encodes several plastid-targeted proteins and hundreds of housekeeping proteins, but it lacks many fundamental genes to maintain itself. Here we report the first two host nucleus-encoded genes for proteins targeted to the nucleomorph, histone H2A and H2B. We identified 20 histone genes from the host nuclear genome, and based on phylogenetic analyses predicted that most of these are derived from the host, but that two histone genes are symbiont-derived. The genes both encode N-terminal extensions resembling PPC targeting signals, further suggesting they function in the nucleomorph. Using green fluorescent protein (GFP) fusion proteins expressed in transformed cells, we confirmed that the putative symbiont H2A and H2B were targeted into the nucleomorph, whereas putative host proteins were localized to the host nucleus. Furthermore, we have developed a method to temporarily synchronize B. natans cells, and confirmed that both host and symbiont histone expression is controlled during the cell cycle. Our findings provide the first evidence of how the nucleomorph may be regulated by host-encoded gene products.
© 2011 Blackwell Publishing Ltd.
DOI: 10.1111/j.1365-2958.2011.07643.x
PMID: 21470316
