Genetic and Biochemical Aspects of Ectoine Biosynthesis in Moderately Halophilic and Halotolerant Methylotrophic Bacteria

Abstract

Problem statement: The cyclic imino acid ectoine is a widely distributed compatible solute synthesizing by halophilic and halotolerant bacteria to prevent osmotic stress at high external salinity. This water-keeping compound is used in a variety of commercial cosmetics and therapeutic products. Approach: Development of integrated, predictive functional model of the metabolic and regulatory netwoks of ectoine-producing microbes is an active area of research. In this article we present a brief overview of the current knowledge on genetic and biochemical aspects of ectoine biosynthesis in aerobic halophilic and halotolerant bacteria utilizing C₁ compounds (methylotrophs). Although enzymology and genetics of the ectoine biosynthesis in methylotrophs are similar to other halophilic bacteria, the regulatory patterns are different. In all methylotrophic bacteria studied, the genes coding for specific enzymes of ectoine biosynthesis: Diaminobutyric Acid (DABA) aminotransferase (EctB), DABA acetyltransferase (EctA) and ectoine synthase (EctC) are organized into ectABC or ectABC-ask, whith is linked to gene encoding Aspartokinase isozyme (Ask). Results: Remarkably, the methylotrophic bacteria possessing a four-gene cluster showed higher halotolerance and accumulated more ectoine than bacteria with a cluster composed of three genes. The DABA acetyltransferases from three methylotrophic species have been comparatively characterized. The properties of the enzymes correlate with eco-physiological and metabolic particularities of the host. Some elements of the regulatory system governing the ectoine pathway operation have been revealed in both methane and methanol utilizing bacteria. In Methylomicrobium alcaliphilum transcription of the ectABC-ask operon is initiated from two σ⁷⁰-like promoters and controlled by the EctR, a MarR-type negative regulator. EctR orthologs were identified in genomes of several heterotrophic halophilic bacteria. Here we present genomic data indicating that similar regulatory system may occur in diverse halophilic and halotolerant bacteria. Conclusion: Currently available data suggest that in methylotrophic bacteria the ectoine biosynthesis pathways are evolutionary well conserved, particularly with respect to the genes and enzymes involved. However, some differences in the ect-gene cluster organization and regulation could be observed.