Use of TnphoA and T7 RNA polymerase to study fimbrial proteins

The acetyltransferase SCO0988 controls positively specialized metabolism and morphological differentiation in the model strains Streptomyces coelicolor and Streptomyces lividans

Streptomycetes are well-known antibiotic producers possessing in their genomes numerous silent biosynthetic pathways that might direct the biosynthesis of novel bio-active specialized metabolites. It is thus of great interest to find ways to enhance the expression of these pathways to discover most needed novel antibiotics. In this study, we demonstrated that the over-expression of acetyltransferase SCO0988 up-regulated the production of specialized metabolites and accelerated sporulation of the weak antibiotic producer, Streptomyces lividans and that the deletion of this gene had opposite effects in the strong antibiotic producer, Streptomyces coelicolor. The comparative analysis of the acetylome of a S. lividans strain over-expressing sco0988 with that of the original strain revealed that SCO0988 acetylates a broad range of proteins of various pathways including BldKB/SCO5113, the extracellular solute-binding protein of an ABC-transporter involved in the up-take of a signal oligopeptide of the quorum sensing pathway. The up-take of this oligopeptide triggers the "bald cascade" that regulates positively specialized metabolism, aerial mycelium formation and sporulation in S. coelicolor. Interestingly, BldKB/SCO5113 was over-acetylated on four Lysine residues, including Lys425, upon SCO0988 over-expression. The bald phenotype of a bldKB mutant could be complemented by native bldKB but not by variant of bldKB in which the Lys425 was replaced by arginine, an amino acid that could not be acetylated or by glutamine, an amino acid that is expected to mimic acetylated lysine. Our study demonstrated that Lys425 was a critical residue for BldKB function but was inconclusive concerning the impact of acetylation of Lys425 on BldKB function.

Characterization of FasG segments required for 987P fimbria-mediated binding to piglet glycoprotein receptors

The 987P fimbriae of enterotoxigenic strains of Escherichia coli bind to both glycoprotein and glycolipid receptors on the brush borders of piglet enterocytes. A mutation in lysine residue 117 of the adhesive subunit FasG [fasG(K117A)] previously shown to abrogate 987P binding to the lipid receptor sulfatide did not affect the interaction with the glycoprotein receptors. Both the fimbriae and the FasG subunits of the wild type and the fasG(K117A) mutant bound to the glycoprotein receptors, confirming that lysine 117 was not required for binding to the glycoprotein receptors. Truncated FasG molecules were used to identify domains required for glycoprotein receptor recognition. At least two segments which did not include lysine117, namely, residues 211 (glutamine) to 220 (serine) and 20 (aspartic acid) to 41 (serine), were shown to be involved in the FasG-glycoprotein receptor interactions by ligand-blotting assays. Changing isoleucine 217 or leucine 215 of FasG to alanine abolished the property of a truncated FasG fusion protein to inhibit 987P recognition of its glycoprotein receptors. Thus, the K117 residue of FasG is required only for binding to the glycolipid receptor, whereas the newly identified hydrophobic residues of the FasG subunit are required specifically for the recognition of the glycoprotein receptor. Taken together, our data indicate that different residues of the FasG adhesin are important in 987P fimbrial binding to sulfatide and glycoprotein receptors, suggesting different mechanisms of interaction.

Cytochrome c(y) of Rhodobacter capsulatus is attached to the cytoplasmic membrane by an uncleaved signal sequence-like anchor

During the photosynthetic growth of Rhodobacter capsulatus, electrons are conveyed from the cytochrome (cyt) bc1 complex to the photochemical reaction center by either the periplasmic cyt c2 or the membrane-bound cyt c(y). Cyt c(y) is a member of a recently established subclass of bipartite c-type cytochromes consisting of an amino (N)-terminal domain functioning as a membrane anchor and a carboxyl (C)-terminal domain homologous to cyt c of various sources. Structural homologs of cyt c(y) have now been found in several bacterial species, including Rhodobacter sphaeroides. In this work, a C-terminally epitope-tagged and functional derivative of R. capsulatus cyt c(y) was purified from intracytoplasmic membranes to homogeneity. Analyses of isolated cyt c(y) indicated that its spectral and thermodynamic properties are very similar to those of other c-type cytochromes, in particular to those from bacterial and plant mitochondrial sources. Amino acid sequence determination for purified cyt c(y) revealed that its signal sequence-like N-terminal portion is uncleaved; hence, it is anchored to the membrane. To demonstrate that the N-terminal domain of cyt c(y) is indeed its membrane anchor, this sequence was fused to the N terminus of cyt c2. The resulting hybrid cyt c (MA-c2) remained membrane bound and was able to support photosynthetic growth of R. capsulatus in the absence of the cyt c(y) and c2. Therefore, cyt c2 can support cyclic electron transfer during photosynthetic growth in either a freely diffusible or a membrane-anchored form. These findings should now allow for the first time the comparison of electron transfer properties of a given electron carrier when it is anchored to the membrane or is freely diffusible in the periplasm.

Rhodobacter capsulatus CycH: a bipartite gene product with pleiotropic effects on the biogenesis of structurally different c-type cytochromes

While searching for components of the soluble electron carrier (cytochrome c2)-independent photosynthetic (Ps) growth pathway in Rhodobacter capsulatus, a Ps- mutant (FJM13) was isolated from a Ps+ cytochrome c2-strain. This mutant could be complemented to Ps+ growth by cycA encoding the soluble cytochrome c2 but was unable to produce several c-type cytochromes. Only cytochrome c1 of the cytochrome bc1 complex was present in FJM13 cells grown on enriched medium, while cells grown on minimal medium contained at various levels all c-type cytochromes, including the membrane-bound electron carrier cytochrome cy. Complementation of FJM13 by a chromosomal library lacking cycA yielded a DNA fragment which also complemented a previously described Ps- mutant, MT113, known to lack all c-type cytochromes. Deletion and DNA sequence analyses revealed an open reading frame homologous to cycH, involved in cytochrome c biogenesis. The cycH gene product (CycH) is predicted to be a bipartite protein with membrane-associated amino-terminal (CycH1) and periplasmic carboxyl-terminal (CycH2) subdomains. Mutations eliminating CyCH drastically decrease the production or all known c-type cytochromes. However, mutations truncating only its CycH2 subdomain always produce cytochrome c1 and affect the presence of other cytochromes to different degrees in a growth medium-dependent manner. Thus, the subdomain CycH1 is sufficient for the proper maturation of cytochrome c1 which is the only known c-type cytochrome anchored to the cytoplasmic membrane by its carboxyl terminus, while CycH2 is required for efficient biogenesis of other c-type cytochromes. These findings demonstrate that the two subdomains of CycH play different roles in the biogenesis of topologically distinct c-type cytochromes and reconcile the apparently conflicting data previously obtained for other species.

Identification of major and minor chaperone proteins involved in the export of 987P fimbriae

The 987P fimbriae of Escherichia coli consist mainly of the major subunit, FasA, and two minor subunits, FasF and FasG. In addition to the previously characterized outer membrane or usher protein FasD, the FasB, FasC, and FasE proteins are required for fimbriation. To better understand the roles of these minor proteins, their genes were sequenced and the predicted polypeptides were shown to be most similar to periplasmic chaperone proteins of fimbrial systems. Western blot (immunoblot) analysis and immunoprecipitation of various fas mutants with specific antibody probes identified both the subcellular localizations and associations of these minor components. FasB was shown to be a periplasmic chaperone for the major fimbrial subunit, FasA. A novel periplasmic chaperone, FasC, which stabilizes and specifically interacts with the adhesin, FasG, was identified. FasE, a chaperone-like protein, is also located in the periplasm and is required for optimal export of FasG and possibly other subunits. The use of different chaperone proteins for various 987P subunits is a novel observation for fimbrial biogenesis in bacteria. Whether other fimbrial systems use a similar tactic remains to be discovered.