Acyl-group specificity of AHL synthases involved in quorum-sensing in Roseobacter group bacteria

Abstract

N-Acylhomoserine lactones (AHLs) are important bacterial messengers, mediating different bacterial traits by quorum sensing in a cell-density dependent manner. AHLs are also produced by many bacteria of the marine Roseobacter group, which constitutes a large group within the marine microbiome. Often, specific mixtures of AHLs differing in chain length and oxidation status are produced by bacteria, but how the biosynthetic enzymes, LuxI homologs, are selecting the correct acyl precursors is largely unknown. We have analyzed the AHL production in Dinoroseobacter shibae and three Phaeobacter inhibens strains, revealing strain-specific mixtures. Although large differences were present between the species, the fatty acid profiles, the pool for the acyl precursors for AHL biosynthesis, were very similar. To test the acyl-chain selectivity, the three enzymes LuxI₁ and LuxI₂ from D. shibae DFL-12 as well as PgaI₂ from P. inhibens DSM 17395 were heterologously expressed in E. coli and the enzymes isolated for in vitro incubation experiments. The enzymes readily accepted shortened acyl coenzyme A analogs, N-pantothenoylcysteamine thioesters of fatty acids (PCEs). Fifteen PCEs were synthesized, varying in chain length from C₄ to C₂₀, the degree of unsaturation and also including unusual acid esters, e.g., 2E,11Z-C18:2-PCE. The latter served as a precursor of the major AHL of D. shibae DFL-12 LuxI₁, 2E,11Z-C18:2-homoserine lactone (HSL). Incubation experiments revealed that PgaI₂ accepts all substrates except C₄ and C₂₀-PCE. Competition experiments demonstrated a preference of this enzyme for C₁₀ and C₁₂ PCEs. In contrast, the LuxI enzymes of D. shibae are more selective. While 2E,11Z-C18:2-PCE is preferentially accepted by LuxI₁, all other PCEs were not, except for the shorter, saturated C₁₀-C₁₄-PCEs. The AHL synthase LuxI₂ accepted only C₁₄ PCE and 3-hydroxydecanoyl-PCE. In summary, chain-length selectivity in AHLs can vary between different AHL enzymes. Both, a broad substrate acceptance and tuned specificity occur in the investigated enzymes.

Keywords: Dinoroseobacter shibae; N-acylhomoserine lactones; Phaeobacter inhibens; fatty acid composition; quorum sensing.

Scheme 1

Biosynthesis of AHLs by ACP-dependent LuxI type enzymes.

Figure 1

Total ion chromatograms of the FAME extracts of A) P. inhibens 2.10, B) P. inhibens DSM17395, C) P. inhibens T5 and D) D. shibae DFL-12. *other compounds.

Scheme 2

Synthesis of N-pantothenoylcysteamine thioesters (PCEs) for feeding experiments with AHL synthases.

Figure 2

Total ion chromatograms of the extracts from competition experiments using recombinant PgaI₂ from P. inhibens with SAM and a mixture of equally concentrated substrates A) 10a–h (saturated chains C₄–C₁₈) and B) 10a–i, 11a–c and 12. Prominent contaminants are indicated by asterisks.