Evolutionary diversification of the RomR protein of the invasive deltaproteobacterium, Bdellovibrio bacteriovorus

Rebecca C Lowry,David S Milner,Steven L Porter,Asmaa M S Al-Bayati,Carey Lambert,R E Sockett,Vanessa I Francis

doi:10.1038/s41598-019-41263-5

Abstract

Bdellovibrio bacteriovorus is a predatory deltaproteobacterium that encounters individual Gram-negative prey bacteria with gliding or swimming motility, and then is able to invade such prey cells via type IVa pilus-dependent mechanisms. Movement control (pili or gliding) in other deltaproteobacteria, such as the pack hunting Myxococcus xanthus, uses a response regulator protein, RomRMx (which dynamically relocalises between the cell poles) and a GTPase, MglAMx, previously postulated as an interface between the FrzMx chemosensory system and gliding or pilus-motility apparatus, to produce regulated bidirectional motility. In contrast, B. bacteriovorus predation is a more singular encounter between a lone predator and prey; contact is always via the piliated, non-flagellar pole of the predator, involving MglABd, but no Frz system. In this new study, tracking fluorescent RomRBd microscopically during predatory growth shows that it does not dynamically relocalise, in contrast to the M. xanthus protein; instead having possible roles in growth events. Furthermore, transcriptional start analysis, site-directed mutagenesis and bacterial two-hybrid interaction studies, indicate an evolutionary loss of RomRBd activation (via receiver domain phosphorylation) in this lone hunting bacterium, demonstrating divergence from its bipolar role in motility in pack-hunting M. xanthus and further evolution that may differentiate lone from pack predators.

Highlights

The predatory deltaproteobacterium, Bdellovibrio bacteriovorus, can invade and replicate intraperiplasmically within other Gram-negative bacteria
Analysis of the B. bacteriovorus romR gene was firstly carried out from the genome sequence published by Rendulic and co-workers[14]
Within the HD100 RomRBd receiver domain (REC) domain, residues required for producing an acidic pocket and for phosphoryl group acceptance were present in the annotation (Fig. 1a)

Summary

Introduction

The predatory deltaproteobacterium, Bdellovibrio bacteriovorus, can invade and replicate intraperiplasmically within other Gram-negative bacteria. B. bacteriovorus strains can, as an alternative, switch to grow host/prey-independently (HI), a process requiring point mutations in Bdellovibrio-specific genes to occur This HI lifestyle can be utilised experimentally to save, culture and analyse non-predatory mutants such as type IVa pilus-defective strains that are incapable of replicating predatorily[3]. MglA-specific GTPase activating protein (GAP), MglB, is predominantly present at the lagging cell pole, preventing the accumulation of GTP bound MglA It is the asymmetric localisation of MglA and MglB that controls cell polarity, and the Frz chemosensory system is responsible for the frequency of M. xanthus cell reversals[6] by inducing the relocalisation of MglA to the opposite pole, which switches the leading and lagging poles (enabling motility reversals)[7,8]. The frz genes are not present within the B. bacteriovorus genome and so whether RomRBd is responding to signals from another system to carry out a distinct role in this intraperiplasmic predator remains unknown

Methods

Results

Conclusion