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Abstract
The secondary messenger cyclic diguanosine monophosphate (cyclic diguanylate or cyclic-di-GMP) is regarded as a key regulator of traits involved in transition of bacteria from a motile to a sessile lifestyle. Cyclic-di-GMP signalling systems are well-studied and characterised in many terrestrial species of bacteria but remain largely unexplored in marine bacteria. This thesis explores the occurrence of cyclic-di-GMP signalling in
marine roseobacters (the Roseobacter clade, Alpha-Proteobacteria), a bacterial group known for its metabolic diversity and ability to associate with eukaryotic hosts in the marine environment. Analysis of genomes of representative members of the Roseobacter clade uncovered the abundance of genes encoding for diguanylate cyclases (DGC) and phosphodiesterases (PDE), enzymes that are involved in cyclic-di-GMP
synthesis and degradation, respectively, as well as genes that encode for proteins involved in cyclic-di-GMP binding. Analysis of the domain structure further revealed that many proteins contain N-terminal sensor domains that potentially link specific environmental cues to the cyclic-di-GMP pathway. Mass spectrometry measurements detected cyclic-di-GMP in all Roseobacter clade strains studied here, which was
consistent with the abundance of synthesis genes. Furthermore many strains apparently possess higher levels of cyclic-di-GMP in planktonic cells than in attached cells, a finding that expands the existing paradigm that high levels of cyclic-di-GMP promote attachment and biofilm formation, while low levels promote motility. The function of specific DGCs and PDEs was also studied in Nautella italica R11, a member of the
Roseobacter clade that lives in association with marine red algae. Attachment and motility were affected by the deletion of specific genes encoding DGCs or PDEs in N. italica R11 suggesting that cyclic-di-GMP-dependent pathways are involved in the regulation of these cellular processes. In general, this thesis provides baseline information on the occurrence of cyclic-di-GMP signalling in marine bacteria, specifically, the marine Roseobacter clade, further expanding our knowledge of the genomic traits of marine roseobacters and contributing to the on-going research on the Roseobacter clade to understand their ecological success in the marine environment.