ECF02

ECF subgroups

ECF02s1 2976 ECF02s2 209 ECF02s3 159

ECF proteins

General description: Members of ECF02 have homology against proteins of original ECF02 (99.29%) and appear in Proteobacteria (100%).

Anti-σ factor: Members of ECF02 are regulated by RseA-like AS factors with one transmembrane helix encoded in position +1. Even though TopCons failed to predict the transmembrane helix in most of the cases, an MSA revealed the presence of one transmembrane helix in most of the proteins encoded in +1 of members of ECF02. RseA-like AS factors take their name from RseA, the AS factor of RpoE in E. coli (ECF02s1) (Missiakas, Mayer, Lemaire, Georgopoulos, & Raina, 1997).

Genomic context conservation: Other conserved domains in the context of ECF02 are associated with translation and protein secretion. We found a 50S ribosome-binding GTPase fused to a KH domain, an elongation factor Tu involved in the translation elongation, a ribonuclease-III-like protein involved in rDNA transcription and rRNA processing, a signal peptidase S24 (Pfam: Peptidase_S24), a signal peptidase S26 and a fumarate reductase (only ECF02s2).

Studied members: Among the characterized members of ECF02 we found RpoE, from E. coli, which plays a key role during stationary phase, metal resistance (Egler, Grosse, Grass, & Nies, 2005), folding and degradation of periplasmic proteins, lipopolysaccharide biogenesis, heat shock response (Rowley, Spector, Kormanec, & Roberts, 2006) and virulence (Smith et al., 2017). Unlike other ECFs, RpoE is essential in E. coli (Hiratsu, Amemura, Nashimoto, Shinagawa, & Makino, 1995), potentially due to the lack of a complete lipopolysaccharide in respect to the closely related Salmonella enterica, where RpoE is not essential (Amar, Pezzoni, Pizarro, & Costa, 2018). RpoE is regulated by its AS factor, RseA, which is inactivated by Regulated Intramembrane Proteolysis (RIP) sequentially carried out by the proteases DegS, RseP and cytoplasmic proteases (Akiyama, Kanehara, & Ito, 2004; Walsh, Alba, Bose, Gross, & Sauer, 2003). DegS is a membrane-bound periplasmic protein able to sense stress signal originated from different unfolded OMP via its PDZ domain (Hasselblatt et al., 2007). RseP contains a pair of permutated PDZ domains in its periplasmic region that controls its activity as site-2 protease (Inaba et al., 2008). Indeed, proteins with PDZ domain appear in the genetic context of ECF02 (1.17 per ECF). The other two proteins in the operon of RpoE - RseB (+2) and RseC (+3), are an extracytoplasmic stabilizer of RseA and an inner membrane positive regulator of RpoE, respectively (Cezairliyan & Sauer, 2007; Missiakas et al., 1997). While RseB-like proteins are conserved in positions +2 of all the subgroups, RseC-like proteins are not conserved in ECF02s3.

RpoE from S. enterica (ECF02s1) is activated by UVA radiation, envelope stress generated by phage infection or hypo-osmotic shock in the absence of periplasmic glycans (OPG) (Amar et al., 2018; Spöring et al., 2018; E. C. Woods & McBride, 2017). RpoE from Burkholderia pseudomallei is induced under high salt concentrations and is required for heat and oxidative stress since it turns on the expression of speG, stabilizing the levels of spermidine (Duangurai, Indrawattana, & Pumirat, 2018). RpoE from Xylella fastidiosa participates in heat shock response due to 21-gene sigmulon (da Silva Neto, Koide, Gomes, & Marques, 2007). Similarly, RpoE from Shewanella oneidensis is essential for growth under suboptimal conditions caused by temperature, salinity, oxidative stress, minimal medium, metals, absence of oxygen, among others, and it typically targets the synthesis of extracytoplasmic and outer membrane components (Barchinger et al., 2016; Dai et al., 2015). Vibrio parahaemolyticus contains an ECF02 (VP2578) involved in cell envelope stress required for intestinal colonization (!!!Haines-Menges et al., 2014!!!).

Interestingly, RpoE1 from Xanthomonas campestris (ECF02s1) is induced under cultivation in minimal medium or plant extract, and it is required by plant pathogenesis since it induces the expression of T3SS (Yang et al., 2018). This function is typical from group ECF32, which is in close evolutionary proximity to ECF02. Therefore, RpoE1 from X. campestris could be the mixture of both systems where the ECF kept the sequence of members of ECF02 but the function of ECF32.

Other members of ECF02 are the AlgU-like proteins — AlgU from Pseudomonas aeruginosa in involved in alginate production leading to transition the mucoid and biofilm phenotypes. AlgU helps to relieve oxidative stress, and it is involved in virulence, β-lactamase production, and repression of flagella biosynthesis. AlgU is induced by antibiotics that block peptidoglycan synthesis as in the case of RpoE from E. coli (Llamas et al., 2014; E. C. Woods & McBride, 2017) (Chevalier et al., 2018; Delgado et al., 2018; Tart, Wolfgang, & Wozniak, 2005). Homologs of AlgU appear in other Pseudomonas species such as P. putida and P. syringae, but also in Azotobacter vinelandii.

Promoter motif conservation: Promoter motifs are well conserved and contain GAACTTT in -35 and GTCT in -10 in agreement with the binding motif of RpoE in E. coli (Rhodius & Mutalik, 2010), Shewanella oneidensis (Barchinger et al., 2016) and original ECF02 (Staroń et al., 2009). Nevertheless, some members of ECF02, such as RpoE from Xylella fastidiosa are not autoregulated (da Silva Neto et al., 2007).

Summary: ECF02 keeps the characteristics of the original group ECF02 (Staroń et al., 2009). ECF02 is one of the key factors contributing to cell envelope homeostasis in Proteobacteria. These ECFs are also involved in biofilm formation since they induce the production of alginate in bacteria from the genus Pseudomonas. Members of ECF02 are negatively regulated by a RseA-like AS factor encoded in position +1, a RseB-like extracytoplasmic negative regulator that protects RseA from degradation and a RseC-like inner membrane positive regulator.