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 LuxI(1) and LuxI(2) from D. shibae DFL-12 as well as PgaI(2) 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 C(4) to C(20), 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 LuxI(1), 2E,11Z-C18:2-homoserine lactone (HSL). Incubation experiments revealed that PgaI(2) accepts all substrates except C(4) and C(20)-PCE. Competition experiments demonstrated a preference of this enzyme for C(10) and C(12) PCEs. In contrast, the LuxI enzymes of D. shibae are more selective. While 2E,11Z-C18:2-PCE is preferentially accepted by LuxI(1), all other PCEs were not, except for the shorter, saturated C(10)-C(14)-PCEs. The AHL synthase LuxI(2) accepted only C(14) 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.