The Twin-Arginine Translocation Pathway in α-Proteobacteria Is Functionally Preserved Irrespective of Genomic and Regulatory Divergence

Abstract

The twin-arginine translocation (Tat) pathway exports fully folded proteins out of the cytoplasm of Gram-negative and Gram-positive bacteria. Although much progress has been made in unraveling the molecular mechanism and biochemical characterization of the Tat system, little is known concerning its functionality and biological role to confer adaptive skills, symbiosis or pathogenesis in the α-proteobacteria class. A comparative genomic analysis in the α-proteobacteria class confirmed the presence of tatA , tatB , and tatC genes in almost all genomes, but significant variations in gene synteny and rearrangements were found in the order Rickettsiales with respect to the typically described operon organization. Transcription of tat genes was confirmed for Anaplasma marginale str. St. Maries and Brucella abortus 2308, two α-proteobacteria with full and partial intracellular lifestyles, respectively. The tat genes of A. marginale are scattered throughout the genome, in contrast to the more generalized operon organization. Particularly, tatA showed an approximately 20-fold increase in mRNA levels relative to tatB and tatC . We showed Tat functionality in B. abortus 2308 for the first time, and confirmed conservation of functionality in A. marginale . We present the first experimental description of the Tat system in the Anaplasmataceae and Brucellaceae families. In particular, in A. marginale Tat functionality is conserved despite operon splitting as a consequence of genome rearrangements. Further studies will be required to understand how the proper stoichiometry of the Tat protein complex and its biological role are achieved. In addition, the predicted substrates might be the evidence of role of the Tat translocation system in the transition process from a free-living to a parasitic lifestyle in these α-proteobacteria.

Secretome of obligate intracellularRickettsia

“…Prior analyses focusing on bacteria (Dilks et al ., 2003) and Alphaproteobacteria (Nunez et al ., 2012) reported Tat systems of Rickettsia spp. lacking TatB proteins.…”

“…lack a gene encoding the TatB protein (Dilks et al ., 2003, Nunez et al ., 2012) . A study on the Tat system of Anaplasma marginale revealed the expression of tatA , tatB and tatC , as well as the ability of recombinant TatA and TatB (but not TatC) to restore Tat function in E .…”

“…A study on the Tat system of Anaplasma marginale revealed the expression of tatA , tatB and tatC , as well as the ability of recombinant TatA and TatB (but not TatC) to restore Tat function in E . coli tat mutant strains (Nunez et al ., 2012) . TatB homologs were also reported for Anaplasma phagocytophilum and Ehrlichia spp., but not for the remaining genera of Rickettsiales ( Wolbachia , Neorickettsia , Orientia , Rickettsia ).…”

“…Despite the presence of Tat systems in Rickettsiales, previous bioinformatics analyses have identified very few predicted Tat substrates (Dilks et al ., 2003, Nunez et al ., 2012) . In support of this, our own examination of in silico approaches (Rose et al ., 2002, Bendtsen et al ., 2005) across Rickettsia genomes identified only one conserved protein containing a putative Tat signal peptide, the ubiquinolcytochrome c reductase (cytochrome b-c1 complex), or Rieske protein (PetA).…”

TolC-Dependent Secretion of an Ankyrin Repeat-Containing Protein of Rickettsia typhi

“…Another study on the twin-arginine translocation (Tat) pathway of Anaplasma marginale also shows the ability of its component genes to retain function in cross-species complementation using E. coli (43). RT0216 transcript expression during R. typhi infection in HeLa cells suggests that its encoded protein may play a role in R. typhi growth and pathogenesis (5).…”