Fig. 4: sinI expression pulse frequency determines quorum sensing response dynamics.

a Different sinI expression pulse frequencies correlate with different quorum-sensing response dynamics, i.e., earlier or later activation of the EPS promoter PwgeA. Data, means ± standard deviations of (left) the same 9 colonies imaged on 3 different days as in Fig. 3d followed for approx. 10 more hours, and of (right) 6 colonies from 2 biological replicates in Fig. 3d followed for approx. 12 more hours; the 3 colonies from the third biological replicate show similar relative differences, but different absolute timing (Supplementary Fig. 12f) that could be due to, e.g., differences in AHL stability on the pad. b (Top) Experimental setup applied to test the effect of different sinR expression levels—and corresponding different sinI expression pulse frequencies—on quorum sensing response dynamics of an S. meliloti AHL indicator strain. (Bottom) The different donor strains need to grow to different optical densities (OD₆₀₀) till their culture supernatants induce the positive feedback on the PsinI-mVenus fusion in the indicator strain, confirming differences in the amount of secreted AHLs as the origin of distinct quorum sensing response dynamics. Data are representative of 2 biological replicates. We could not observe an effect on PwgeA, probably due to substantial loss of AHLs during harvest and sterile filtration of the supernatants. c Model illustrating frequency modulation of the S. meliloti quorum sensing response.