Abstract
Rickettsiales are obligate intracellular bacteria originally found in metazoans, but more recently recognized as widespread endosymbionts of various protists. One genus was detected also in several green algae, but reports on rickettsialean endosymbionts in other algal groups are lacking. Here we show that several distantly related eustigmatophytes (coccoid algae belonging to Ochrophyta, Stramenopiles) are infected by Candidatus Phycorickettsia gen. nov., a new member of the family Rickettsiaceae. The genome sequence of Ca. Phycorickettsia trachydisci sp. nov., an endosymbiont of Trachydiscus minutus CCALA 838, revealed genomic features (size, GC content, number of genes) typical for other Rickettsiales, but some unusual aspects of the gene content were noted. Specifically, Phycorickettsia lacks genes for several components of the respiration chain, haem biosynthesis pathway, or c-di-GMP-based signalling. On the other hand, it uniquely harbours a six-gene operon of enigmatic function that we recently reported from plastid genomes of two distantly related eustigmatophytes and from various non-rickettsialean bacteria. Strikingly, the eustigmatophyte operon is closely related to the one from Phycorickettsia, suggesting a gene transfer event between the endosymbiont and host lineages in early eustigmatophyte evolution. We hypothesize an important role of the operon in the physiology of Phycorickettsia infection and a long-term eustigmatophyte-Phycorickettsia coexistence.
Highlights
Eukaryotic cells are inherently endowed with the capability of harbouring prokaryotic endosymbionts [1,2,3]
Inspection of an initial assembly of 454 reads obtained from total DNA isolated from a culture of T. minutus CCALA 838 revealed that some of the largest scaffolds exhibit apparent similarity to genes from Rickettsiaceae and are presumably derived from a bacterium representing a new lineage in the family based on a phylogenetic analysis of the 16S ribosomal RNA (rRNA) gene
We reasoned that this bacterium may be more generally associated with eustigmatophytes, so we designed PCR primers to amplify the 16S ribosomal DNA (rDNA) of this bacterium or its close relatives, but not from other bacteria including other members of the Rickettsiaceae
Summary
Eukaryotic cells are inherently endowed with the capability of harbouring prokaryotic endosymbionts [1,2,3]. The known host range has recently been extended to several distantly related taxa of green algae [4, 14,15,16] harbouring bacteria representing two closely related lineages, Candidatus Megaira polyxenophila and Ca. Megaira subclade B, which are promiscuous partners of additional host types (ciliates, vamyrellids, haplosporidians, or corals) [4, 12, 17, 18]. Megaira subclade B, which are promiscuous partners of additional host types (ciliates, vamyrellids, haplosporidians, or corals) [4, 12, 17, 18] Such a broad occurrence suggests that endosymbionts belonging to Ca. Megaira are loosely associated with their hosts and can be transferred to new, phylogenetically remote hosts
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