The fine structure of striated microtubules and sleeve bodies in several species of Anabaena

Structural Determinants and Their Role in Cyanobacterial Morphogenesis

“…Life 2020, 10, x FOR PEER REVIEW 22 of 33 only in Anabaena [215] and in Cyanothece [220]. These finer filaments were observed in all areas of the cytoplasm and similar findings were recently made in our lab (unpublished), after artefact freethinning of frozen-hydrated Anabaena cells with cryo-focused ion beam (FIB) milling [221] and subsequent cryoET.…”

“…Thin sections of Nostoc strains revealed an even more intricate arrangement of tubules, consisting of an amorphous, ~1 µm long base plate parallel to the septum and numerous microtubule-like filaments perpendicular to it, protruding towards the cell center [ 214 ]. A similar complex could be visualized in Anabaena ([ 215 ] and reviewed in [ 216 ]); however, the described architecture shows strong similarities to phycobilisome arrays bound to thylakoid membranes, which were recently resolved in a native state in Synechocystis with cryo-electron tomography (cryoET) [ 217 ]. In Anabaena , it was further speculated that microtubular filaments could be important for the positioning of carboxysomes [ 215 ], whereas, the reported striated microtubules and sleeve bodies ( Figure 5 A) more resemble a membranous compartment or vesicles if re-analyzed with today’s knowledge and to a lesser extent cytoskeletal features [ 215 ].…”

“…A similar complex could be visualized in Anabaena ([ 215 ] and reviewed in [ 216 ]); however, the described architecture shows strong similarities to phycobilisome arrays bound to thylakoid membranes, which were recently resolved in a native state in Synechocystis with cryo-electron tomography (cryoET) [ 217 ]. In Anabaena , it was further speculated that microtubular filaments could be important for the positioning of carboxysomes [ 215 ], whereas, the reported striated microtubules and sleeve bodies ( Figure 5 A) more resemble a membranous compartment or vesicles if re-analyzed with today’s knowledge and to a lesser extent cytoskeletal features [ 215 ]. In contrast, the finding of tubular structures bound to the cytoplasmic membrane in two Nostoc strains is even more remarkable nowadays [ 218 ], as they show striking similarities to the phage tail-like apparatus of the bacterial type VI secretion system, which was structurally discovered more than 30 years later and has not yet been identified in cyanobacteria [ 219 ].…”

“…In contrast, the finding of tubular structures bound to the cytoplasmic membrane in two Nostoc strains is even more remarkable nowadays [ 218 ], as they show striking similarities to the phage tail-like apparatus of the bacterial type VI secretion system, which was structurally discovered more than 30 years later and has not yet been identified in cyanobacteria [ 219 ]. The second prominent observations, which were termed microfilaments, could be visualized only in Anabaena [ 215 ] and in Cyanothece [ 220 ]. These finer filaments were observed in all areas of the cytoplasm and similar findings were recently made in our lab (unpublished), after artefact free-thinning of frozen-hydrated Anabaena cells with cryo-focused ion beam (FIB) milling [ 221 ] and subsequent cryoET.…”

“…Shown are 13.5 nm thick slices. Figure 5 A is reprinted from “The fine structure of striated microtubules and sleeve bodies in several species of Anabaena ” [ 215 ], vol. 57, 1976, by Thomas E. Jensen and Robert P. Ayala with permission from Elsevier.…”