Divergent light-harvesting complexes
نویسندگان
چکیده
‡Current address: University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195-7940, USA. †Co first authors. Approximately one-half of the total primary production on Earth occurs in the oceans [1]. Although hundreds of species contribute to oceanic productivity, members of two cyanobacterial genera – Prochlorococcus [2,3] and Synechococcus [4] – are responsible for a significant fraction of the primary production in open ocean waters of the subtropical and tropical Atlantic and Pacific [5,6]. In recognition of their global significance, the genomes of three representative isolates of these two groups have recently been sequenced as part of the DOE Microbial Genome Project (http://www.jgi.doe.gov/ JGI_microbial/html/index.html). A characteristic feature of the majority of extant cyanobacteria is their supramolecular lightharvesting antenna, the PHYCOBILISOME (see Glossary). Although Prochlorococcus and marine A-cluster Synechococcus form sister clades within the cyanobacteria [7,8], Prochlorococcus has evolved to use a chlorophyll a2/b2 (Chl a2/b2) light-harvesting complex rather than the phycobilisome (Fig. 1; Box 1). Phylogenetic trees based on 16S rDNA sequences suggest a rapid diversification of these two genera from a common phycobilisome-containing ancestor [9]. The close phylogenetic relationship between Prochlorococcus and Synechococcus combined with the recent availability of whole genome sequences makes it possible to examine how the evolution of a key protein complex – the light-harvesting antenna – has proceeded along different paths in these two genera and might contribute to their ability to dominate specific oceanic habitats. Here, we describe major characteristics of the two antennas and present a scenario for the evolution of a Prochlorococcus Chl a/b-based antenna from a Chl abinding, iron-stress-induced protein (IsiA or CP43′). This latter chlorophyll-binding protein might have been maintained constitutively in an ancestral cyanobacterium inhabiting iron-limited oceans if it provided a selective advantage over the phycobilisome complex in a particular oceanic environment. We then focus on the key environmental factors, light and nitrogen, and examine how differences in the major light-harvesting antennas of extant Prochlorococcus and Synechococcus contribute to niche differentiation in these cyanobacteria.
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تاریخ انتشار 2002