SUCROSE BIOSYNTHESIS IN MARINE CYANOBACTERIA

PEREZ CENCI M; SALERNO G.L.

Mar Del Plata

Congreso; VIIICongreso de Microbiología General.; 2012

Marine picocyanobacteria are the most abundant photosynthetic organisms on Earth, with two genera, Prochlorococcus and Synechococcus numerically dominating most oceanic waters. The synthesis and accumulation of compatible solutes represent an essential part of their salt acclimation strategy. Sucrose (Suc) serves as an intermediate in central carbon metabolism, but it may have an additional function in osmoregulation. In plants and cyanobacteria it has been reported that Suc, one of the most common non-reducing disaccharides used as compatible solute, is synthesized through a two-step pathway involving SPS (U/ADP-glucose:D-fructose-6-P-2-a-D-glucosyltransferase) and SPP (sucrose-6-P-phospho-hydrolase). Due to Synechococcus and Prochlorococcus importance as the main primary producers in the oceans, 31 genomes of these cyanobacteria have been totally sequenced. Cell Suc accumulation has been reported in Synechococcus sp. strain WH7803 and PCC 7002, and in Prochlorococcus sp. strains MIT9312, MIT9313, NATL2A, and SS120. In the case of Prochlorococcus, Suc represents the major compatible solute. However, the Suc metabolisms related genes have not been characterized. We performed searches for putative sequences encoding proteins involved in Suc biosynthesis. In 27 among the 31 Synechococcus and Prochlorococcus genomes we retrieved homologs to Suc protein genes. All Prochorococcus and most Synechococcus lack SPP encoding sequences. Two different SPS modular arrangements were found: i) bidomainal SPSs (SPS:GTD-PHD), where the N-terminal region (GTD, glusosyltransferase domain) is fused to a phosphatase domain (PHD); and ii) a minimal SPS structure (SPS:GTD). Prochlorococcus with a genome size smaller than 1.86 Mb have SPS:GTD sequences. Remarkably, similarly to most Synechococcus, two Prochlorococcus strains (MIT9303 and MIT9313, with genome of 2.4 and 2.68 Mb, respectively) have sequences encoding putative bidomainal SPSs. Functional characterization of SPS encoding genes from Prochlorococcus MED4 and Synechococcus sp. PCC 7002 by heterologous expression in Escherichia coli, revealed that both SPS types (SPS:GTD-PHD and SPS:GTD) exhibited SPS activity. Gene cluster organization regarding the localization of putative SPS encoding genes is conserved in most Synechococcus and Prochlorococcus genomes. Phylogenetic analysis indicates that ancestral SPS:GTD-PHD may have originated the Prochlorococcus SPS:GTD proteins. SPS sequences from Synechococcus strains PCC 7002, PCC 7001, and WH5701 group in a separate cluster and do not maintain the genomic organization, which suggest that Suc biosynthesis genes may have been acquired independently at least in two different evolutionary events. Interestingly, Prochlorococcus strains, organisms with a particularly reduced set of protein-coding genes, still retain Suc biosynthesis to cope with salt acclimation. Supported by CONICET (PIP 134 and CEBB-INBA), UNMdP (EXA552/11), and FIBA.