The LacI-Family Transcription Factor, RbsR, Is a Pleiotropic Regulator of Motility, Virulence, Siderophore and Antibiotic Production, Gas Vesicle Morphogenesis and Flotation in Serratia.

Chin M Lee,Rita E Monson,George P C Salmond,Rachel M Adams

doi:10.3389/fmicb.2017.01678

Abstract

Gas vesicles (GVs) are proteinaceous, gas-filled organelles used by some bacteria to enable upward movement into favorable air/liquid interfaces in aquatic environments. Serratia sp. ATCC39006 (S39006) was the first enterobacterium discovered to produce GVs naturally. The regulation of GV assembly in this host is complex and part of a wider regulatory network affecting various phenotypes, including antibiotic biosynthesis. To identify new regulators of GVs, a comprehensive mutant library containing 71,000 insertion mutants was generated by random transposon mutagenesis and 311 putative GV-defective mutants identified. Three of these mutants were found to have a transposon inserted in a LacI family transcription regulator gene (rbsR) of the putative ribose operon. Each of these rbsR mutants was GV-defective; no GVs were visible by phase contrast microscopy (PCM) or transmission electron microscopy (TEM). GV deficiency was caused by the reduction of gvpA1 and gvrA transcription (the first genes of the two contiguous operons in the GV gene locus). Our results also showed that a mutation in rbsR was highly pleiotropic; the production of two secondary metabolites (carbapenem and prodigiosin antibiotics) was abolished. Interestingly, the intrinsic resistance to the carbapenem antibiotic was not affected by the rbsR mutation. In addition, the production of a siderophore, cellulase and plant virulence was reduced in the mutant, whereas it exhibited increased swimming and swarming motility. The RbsR protein was predicted to bind to regions upstream of at least 18 genes in S39006 including rbsD (the first gene of the ribose operon) and gvrA. Electrophoretic mobility shift assays (EMSA) confirmed that RbsR bound to DNA sequences upstream of rbsD, but not gvrA. The results of this study indicate that RbsR is a global regulator that affects the modulation of GV biogenesis, but also with complex pleiotropic physiological impacts in S39006.

Highlights

The capacity to move is an important ecological adaptation in bacteria
sp. ATCC39006 (S39006) strains were grown at 30◦C and Escherichia coli strains were grown at 37◦C in sealed plastic universals containing Lysogeny Broth (LB; 5 g l−1 yeast extract, 10 g l−1 tryptone, and 5 g l−1 NaCl) or on solid LB agar plate (LBA) supplemented with 1.5% (w/v) agar
Transposons insertions were identified in gvpA1 and gvpN— two genes that lie within the gas vesicle biosynthetic cluster and which we showed previously to be essential for robust gas vesicle formation (Tashiro et al, 2016)

Summary

Introduction

Bacteria are exposed to constantly changing environments and so mobility provides potential advantages for survival. GVs are proteinaceous gas-filled intracellular organelles that facilitate buoyancy. They are synthesized by aquatic Eubacteria and Archaea (Pfeifer, 2012). GVs are spindle- or cylinder-shaped structures comprised of a thin proteinaceous wall (Pfeifer, 2012). The wall of the GV is freely permeable to dissolved gases such as oxygen, carbon dioxide, nitrogen, and methane that are present in the environment (Walsby, 1982). GVs reduce overall cell density and thereby enable bacterial cells to float and colonize air-liquid interfaces for enhanced fitness. ATCC39006 (S39006), was reported (Ramsay et al, 2011; Tashiro et al, 2016)

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Conclusion