Functional dissection of the three-domain SepJ protein joining the cells in cyanobacterial trichomes

Heterocyst-forming cyanobacteria grow as filaments of cells (trichomes) in which, under nitrogen limitation, two interdependent cell types, the vegetative cells performing oxygenic photosynthesis and the nitrogen-fixing heterocysts, exchange metabolites and regulatory compounds. SepJ is a protein conspicuously located at the cell poles in the intercellular septa of the filaments that has three well-defined domains: an N-terminal coiled-coil domain, a central linker and a C-terminal permease domain. Mutants of Anabaena sp. strain PCC 7120 carrying SepJ proteins with specific deletions showed that, whereas the linker domain is dispensable, the coiled-coil domain is required for polar localization of SepJ, filament integrity, normal intercellular transfer of small fluorescent tracers and diazotrophy. An Anabaena strain carrying the SepJ protein from the filamentous, non-heterocyst-forming cyanobacterium Trichodesmium erythraeum, which lacks the linker domain, made long filaments in the presence of combined nitrogen but fragmented extensively under nitrogen deprivation and did not grow diazotrophically. In contrast, a chimera made of the Trichodesmium coiled-coil domain and the Anabaena permease allowed heterocyst differentiation and diazotrophic growth. Thus, SepJ provides filamentous cyanobacteria with a cell-cell anchoring function, but the permease domain has evolved in heterocyst formers to provide intercellular molecular exchange functions required for diazotrophy. © 2010 Blackwell Publishing Ltd.

We thank Gustaf Sandh and Birgitta Bergman for a sample of T. erythraeum genomic DNA. Use of DNA sequences from the National Center for Biotechnology Information (USA), DOE-Joint Genome Institute (USA) and Kazusa DNA Research Institute (Japan) databases is acknowledged. Work in Seville was supported by Grants BFU2008-03811 from Ministerio de Ciencia y Tecnología, co-financed by FEDER, and CVI1896 from Junta de Andalucía (Spain). The project also used equipment purchased with grants to C.W.M. from the Wellcome Trust and Biotechnology and Biological Sciences Research Council (UK).

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