Haloarchaea and the formation of gas vesicles.

Felicitas Pfeifer

doi:10.3390/life5010385

Abstract

Halophilic Archaea (Haloarchaea) thrive in salterns containing sodium chloride concentrations up to saturation. Many Haloarchaea possess genes encoding gas vesicles, but only a few species, such as Halobacterium salinarum and Haloferax mediterranei, produce these gas-filled, proteinaceous nanocompartments. Gas vesicles increase the buoyancy of cells and enable them to migrate vertically in the water body to regions with optimal conditions. Their synthesis depends on environmental factors, such as light, oxygen supply, temperature and salt concentration. Fourteen gas vesicle protein (gvp) genes are involved in their formation, and regulation of gvp gene expression occurs at the level of transcription, including the two regulatory proteins, GvpD and GvpE, but also at the level of translation. The gas vesicle wall is solely formed of proteins with the two major components, GvpA and GvpC, and seven additional accessory proteins are also involved. Except for GvpI and GvpH, all of these are required to form the gas permeable wall. The applications of gas vesicles include their use as an antigen presenter for viral or pathogen proteins, but also as a stable ultrasonic reporter for biomedical purposes.

Highlights

The moderately to extremely halophilic Archaea (Haloarchaea) are adapted to salty environments with sodium chloride concentrations ranging from 1 M up to saturation at 5.3 M
The gvp2 gene cluster resides on the mini-chromosome, pNRC200, in NRC-1, which contains a 145-kbp duplication of pNRC100, including a copy of gvp1, whereas c-vac in R1 is located on plasmid pHS3, similar to c-vac in Hbt. salinarum PHH1 (Table 1)
The GvpE/GvpD regulatory system is often not involved in the transfer of environmental signals to the respective vac region. It appears that both regulatory proteins are mainly required to enhance the gas vesicle formation under standard conditions in Hbt. salinarum and Hfx. mediterranei

Summary

Introduction

The moderately to extremely halophilic Archaea (Haloarchaea) are adapted to salty environments with sodium chloride concentrations ranging from 1 M up to saturation at 5.3 M. The gvp gene cluster resides on the mini-chromosome, pNRC200, in NRC-1, which contains a 145-kbp duplication of pNRC100, including a copy of gvp, whereas c-vac in R1 is located on plasmid pHS3, similar to c-vac in Hbt. salinarum PHH1 (Table 1). Differences are seen with respect to TFB (Table 1) [26] All of these variations in tfb and tbp genes might explain the differences in the amount of gas vesicles in Hbt. salinarum PHH1 and NRC-1 when both are grown under non-standard conditions. All three natural isolates produce cylinder-shaped gas vesicles in late exponential growth due to the expression of c-vac, similar to Hbt. salinarum PHH4. The lack of the pHH1-type plasmids implies that p-vac might be derived from a lateral gene transfer event

Regulation at the Level of Transcription Involves GvpE and GvpD

Environmental Factors Influencing Transcription

Gas Vesicle Wall and Mutations Affecting the Shape

Application of Gas Vesicles

Findings

Conclusions