A Unique Combination of Two Different Quorum Sensing Systems in the β‑Rhizobium Cupriavidus taiwanensis

Cupriavidus taiwanensis LMG19424, a β-rhizobial symbiont of Mimosa pudica, harbors phc and tqs quorum sensing (QS), which are the homologous cell–cell communication systems previously identified from the plant pathogen Ralstonia solanacearum and the human pathogen Vibrio cholerae, respectively. However, there has been no experimental evidence reported that these QS systems function in C. taiwanensis LMG19424. We identified (R)-methyl 3-hydroxymyristate (3-OH MAME) and (S)-3-hydroxypentadecan-4-one (C15-AHK) as phc and tqs QS signals, respectively, and characterized these QS systems. The expression of the signal synthase gene phcB and tqsA in E. coli BL21­(DE3) resulted in the high production of 3-OH MAME and C15-AHK, respectively. Their structures were elucidated by comparison of EI-MS data and GC/chiral LC retention times with synthetic standards. The deletion of phcB reduced cell motility and increased biofilm formation, and the double deletion of phcB/tqsA caused the accumulation of the metal chelator coproporphyrin III in its mutant culture. Although the deletion of phcB and tqsA slightly reduced its ability to nodulate on aseptically grown seedlings of M. pudica, there was no significant difference in nodule formation between LMG19424 and its QS mutants when commercial soils were used. Taken together, this is the first example of the simultaneous production of 3-OH MAME/C15-AHK as QS signals in a bacterial species, and the importance of the phc/tqs QS systems in the saprophytic stage of C. taiwanensis LMG19424 is suggested.