Abstract
Patellamides are highly bioactive compounds found along with other cyanobactins in the symbiosis between didemnid ascidians and the enigmatic cyanobacterium Prochloron. The biosynthetic pathway of patellamide synthesis is well understood, the relevant operons have been identified in the Prochloron genome and genes involved in patellamide synthesis are among the most highly transcribed cyanobacterial genes in hospite. However, a more detailed study of the in vivo dynamics of patellamides and their function in the ascidian-Prochloron symbiosis is complicated by the fact that Prochloron remains uncultivated despite numerous attempts since its discovery in 1975. A major challenge is to account for the highly dynamic microenvironmental conditions experienced by Prochloron in hospite, where light-dark cycles drive rapid shifts between hyperoxia and anoxia as well as pH variations from pH ~6 to ~10. Recently, work on patellamide analogues has pointed out a range of different catalytic functions of patellamide that could prove essential for the ascidian-Prochloron symbiosis and could be modulated by the strong microenvironmental dynamics. Here, we review fundamental properties of patellamides and their occurrence and dynamics in vitro and in vivo. We discuss possible functions of patellamides in the ascidian-Prochloron symbiosis and identify important knowledge gaps and needs for further experimental studies.
Highlights
Discovery of Prochloron and the Ascidian-Prochloron SymbiosisProchloron are unicellular (~10–25 μm), spherical cyanobacteria with appressed thylakoids that engage in symbiosis withtropical didemnid ascidians and, less commonly, some marine invertebrates such as porifera and holothurians [1]
These promising reports indicate that stable long-term cultivation of Prochloron is within reach, which would be instrumental for more detailed studies of some of the many knowledge gaps in understanding the cell biology and the ecological and physiological role of bioactive secondary metabolites in Prochloron and their potential role in regulating the ascidian-Prochloron symbiosis
Patellamides are bioactive cyclic peptides that have been intensively studied with respect to their structural dynamics, pharmacological applications, metal ion ( copper(II)) solution chemistry, and the reactivity of the corresponding copper(II)-patellamide complexes
Summary
Prochloron are unicellular (~10–25 μm), spherical cyanobacteria with appressed thylakoids that engage in symbiosis with (sub)tropical didemnid ascidians (tunicates) and, less commonly, some marine invertebrates such as porifera and holothurians [1]. Together with other features of its photosynthetic apparatus such as appressed thylakoids, Prochloron was initially considered a missing link in chloroplast evolution and thought to represent an oxygenic phototrophic bacterium, from which green plastids had evolved This inspired the name Prochloron and led to its classification as a prochlorophyte [12], a bacterial order that encompasses other cyanobacteria, i.e., Prochlorococcus and Prochlorothrix, with similar, unusual photopigmentation. Molecular analysis of the 16S rRNA gene and the gene coding for chlorophyll a oxygenase (CAO) in enrichment cultures showed high similarity to Prochloron didemni, and cells produced characteristic secondary metabolites such as patellamide E and ulithiacyclamide [16] These promising reports indicate that stable long-term cultivation of Prochloron is within reach, which would be instrumental for more detailed studies of some of the many knowledge gaps in understanding the cell biology and the ecological and physiological role of bioactive secondary metabolites in Prochloron and their potential role in regulating the ascidian-Prochloron symbiosis.
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