The Bdellovibrio bacteriovorus twin-arginine transport system has roles in predatory and prey-independent growth

Chien-Yi Chang,Pratik Jagtap,Rob Till,Michael Capeness,Shin-Ichi Aizawa,Machi Kanna,R Elizabeth Sockett,Laura Hobley,William Burtt

doi:10.1099/mic.0.052449-0

Abstract

Bdellovibrio bacteriovorus grows in one of two ways: either (i) predatorily [in a host-dependent (HD) manner], when it invades the periplasm of another Gram-negative bacterium, exporting into the prey co-ordinated waves of soluble enzymes using the prey cell contents for growth; or (ii) in a host-independent (HI) manner, when it grows (slowly) axenically in rich media. Periplasmic invasion potentially exposes B. bacteriovorus to extremes of pH and exposes the need to scavenge electron donors from prey electron transport components by synthesis of metalloenzymes. The twin-arginine transport system (Tat) in other bacteria transports folded metalloenzymes and the B. bacteriovorus genome encodes 21 potential Tat-transported substrates and Tat transporter proteins TatA1, TatA2 and TatBC. GFP tagging of the Tat signal peptide from Bd1802, a high-potential iron-sulfur protein (HiPIP), revealed it to be exported into the prey bacterium during predatory growth. Mutagenesis showed that the B. bacteriovorus tatA2 and tatC gene products are essential for both HI and HD growth, despite the fact that they partially complement (in SDS resistance assays) the corresponding mutations in Escherichia coli where neither TatA nor TatC are essential for life. The essentiality of B. bacteriovorus TatA2 was surprising given that the B. bacteriovorus genome encodes a second tatA homologue, tatA1. Transcription of tatA1 was found to be induced upon entry to the bdelloplast, and insertional inactivation of tatA1 showed that it significantly slowed the rates of both HI and HD growth. B. bacteriovorus is one of a few bacterial species that are reliant on a functional Tat system and where deletion of a single tatA1 gene causes a significant growth defect(s), despite the presence of its tatA2 homologue.

Highlights

The twin-arginine translocation (Tat) system transports folded proteins across the cytoplasmic membrane and is found in a wide variety of bacteria, whilst homologous systems are found in both archaea and eukaryotes (Sargent, 2007a; Yuan et al, 2010)
The Tat transport system is important in many bacteria as it transports folded proteins across the cytoplasmic membrane, including many proteins involved in redox reactions, metabolism and metal acquisition (Berks et al, 2005; Palmer et al, 2005)
The growing HD B. bacteriovorus carries out extensive transport of proteins required for the predatory hydrolysis of the prey contents and the HI B. bacteriovorus metabolizes complex peptonebased media; we hypothesized that the Tat transport system may play a role in predation and/or axenic growth

Summary

Introduction

The twin-arginine translocation (Tat) system transports folded proteins across the cytoplasmic membrane and is found in a wide variety of bacteria, whilst homologous systems are found in both archaea and eukaryotes (Sargent, 2007a; Yuan et al, 2010). Proteins transported by the Tat. 052449 G 2011 SGM Printed in Great Britain system in bacteria have a consensus twin arginine motif S/T-R-R-X-F-L-K (where X is any polar amino acid), a hydrophobic region and an A-x-A cleavage recognition motif in their N-terminal leading sequence (Berks, 1996; Stanley et al, 2000). Tat-transported proteins are typically involved in metabolism, redox reactions, metal acquisition and cell envelope maintenance (Berks et al, 2005; Palmer et al, 2005) and are involved in pathogenesis, symbiosis, quorum sensing and motility (Sargent, 2007a; Stevenson et al, 2007).

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