TonB-dependent transporters and their occurrence in cyanobacteria

Abstract

Background: Different iron transport systems evolved in Gram-negative bacteria during evolution. Most of the transport systems depend on outer membrane localized TonB-dependent transporters (TBDTs), a periplasma-facing TonB protein and a plasma membrane localized machinery (ExbBD). So far, iron chelators (siderophores), oligosaccharides and polypeptides have been identified as substrates of TBDTs. For iron transport, three uptake systems are defined: the lactoferrin/transferrin binding proteins, the porphyrin-dependent transporters and the siderophore-dependent transporters. However, for cyanobacteria almost nothing is known about possible TonB-dependent uptake systems for iron or other substrates.

FdTonB is involved in the photoregulation of cellular morphology during complementary chromatic adaptation in Fremyella diplosiphon

“…PCC 6803 (Cartron et al, 2006). Notably, although the definitive answer to whether additional TonB-related proteins or proteins such as FeoB exist in F. diplosiphon awaits the availability of the full genome sequence, TonB-family proteins appear to be present as single members, or rarely two members, in related cyanobacteria based on sequence similarity searches (data not shown; Mirus et al, 2009).…”

“…Using TonB-dependent outer-membrane receptors, ironbound siderophores transport through the outer membranes into the cytoplasm, mediated by an ABC transporter present in the cytoplasmic membrane (Schalk et al, 2004). Although the presence of sequences with similarity to TonB-dependent transporters has been recently reported for a number of cyanobacterial genomes (Mirus et al, 2009), the functions of TonB-dependent receptors have not been studied extensively in cyanobacteria. Several proteins with homology to siderophore receptors have been characterized in the freshwater cyanobacterium Synechocystis sp.…”

“…These proteins also are larger than average, including related TonB-family proteins in Nostoc sp. PCC 7120 (All5036; 470 aa in length; Mirus et al, 2009), Nostoc punctiforme PCC 73102 (480 aa), Anabaena variabilis ATCC 29413 (507 aa) and Nodularia spumigena CCY9414 (526 aa). All of these proteins contained the conserved SXXXH motif of the transmembrane domain and the conserved Y and SSG motif in the TBRD (Fig.…”

New insights into iron acquisition by cyanobacteria: an essential role for ExbB-ExbD complex in inorganic iron uptake

“…However, the siderophoremediated iron uptake pathway described in non-photosynthetic bacteria has not been fully confirmed in cyanobacterial species (Stevanovic et al, 2012). The following observations are pertinent: (i) although cyanobacteria possess an OM and are commonly considered as Gram-negative bacteria, their cell envelopes are partly characteristic of Gram-positive bacteria, which do not possess the TonB-ExbB-ExbD system (Hoiczyk and Hansel, 2000); (ii) while some species produce strong siderophores, most cyanobacteria do not (Ito and Butler, 2005;Hopkinson and Morel, 2009;Mirus et al, 2009);and (iii) some cyanobacteria can use the iron bound to siderophores of other organisms (Kranzler et al, 2011). Hopkinson and Morel (2009) suggested that the role of siderophores in marine environments is probably overestimated but could reach no definitive conclusion, because the iron acquisition mechanisms of cyanobacteria are poorly understood.…”

The Outer Membrane TolC-like Channel HgdD Is Part of Tripartite Resistance-Nodulation-Cell Division (RND) Efflux Systems Conferring Multiple-drug Resistance in the Cyanobacterium Anabaena sp. PCC7120

“…Redundant sequences were removed with cdhit (21). The remaining 7,901 RND and 12,918 HlyD sequences were clustered with CLANS (22,23). Sequences from functionally characterized RND family proteins were assigned (protein name followed by UniProt ID): CnrA (CNRA_RALME), AcrB (ACRB_ ECOLI), CusA (CUSA_ECOLI), CzcA (B1J3Z9_PSEPW), AcrF (ACRF_ECOLI), AcrD (ACRD_ECOLI), ArpB (ARPB_PSEPU), TtgB (TTGB_PSEPU), MdtB (multidrug tolerance B)/YegN (MDTB_ECOLI), MdtF/YhiV (MDTF_ECOLI), MdtC (MDTC_ ECOLI), MexB (MEXB_PSEAE), MexD (Q9HVI9_PSEAE), MexF (multidrug efflux transporter F) (Q9I0Y8_PSEAE), MexI (Q9HWH4_PSEAE), EmhB (Q6V6X8_PSEFL), EefB (Q8GC83_ ENTAE).…”