New and Unexpected Insights into the Modulation of LuxR-Type Quorum Sensing by Cyclic Dipeptides

Abstract

Quorum sensing (QS) is under the control of N-acylated L-homoserine lactones (AHLs) and their cognate receptors (LuxR-type proteins) in Gram-negative bacteria, and plays a major role in mediating host-bacteria interactions by these species. Certain cyclic dipeptides (2,5-diketopiperazines, DKPs) have been isolated from bacteria and reported to activate or inhibit LuxRtype proteins in AHL biosensor strains, albeit at significantly higher concentrations than native lactones. These reports have prompted the proposal that DKPs represent a new class of QS signals, and potentially even interspecies or interkingdom signals; their mechanisms of action and physiological relevance, however, remain unknown. Here, we describe a library of synthetic DKPs that was designed to (1) determine the structural features necessary for LuxR-type protein activation and inhibition, and (2) probe their mechanisms of action. These DKPs, along with several previously reported natural DKPs, were screened in bacterial reporter gene assays. In contrast to previous reports, the native DKPs failed to exhibit either antagonistic or agonistic activities in these assays. However, non-natural halogenated cyclo(L-Pro-L-Phe) derivatives were capable of inhibiting luminescence in V. fischeri. Interestingly, additional experiments revealed that these DKPs do not compete with the natural lactone signal, OHHL, to inhibit luminescence. Together, these data suggest that DKPs are not QS signals in the bacteria examined in this study. Although these compounds can influence QS-regulated outcomes, we contend that they do not do so through direct interaction with LuxR-type proteins. This work serves to refine the lexicon of naturally occurring QS signals used by Gram-negative bacteria.

Inhibition of biofilm in Bacillus amyloliquefaciens Q-426 by diketopiperazines

“…Although there are different opinions on the roles of DKPs play in QS systems (Campbell et al 2009), it has also been well documented that certain DKPs were found to be capable of activating or antagonizing LuxR-based QS systems (Degrassi et al 2002;Holden et al 1999;Tommonaro et al 2012). Klose (2006) reported that L-cyclo(Pro-Phe) could stimulate the expression of major virulence factors in V. cholerae.…”

The Function of Cyclo(Phe-Pro) in Gene Expression of Vibrio Harveyi

“…Since cFP had the same apparent effect on these two strains as the others, any signaling though quorum sensing would have to operate through a different signal/sensor system. The need to find the signal/sensor system cFP uses does not conflict with the finding by Campbell et al (2009) that cFP signaling works through LuxR.…”

“…Cyclic dipeptides are also found in fungi, and produce a number of effects in animals and plants it has been suggested that they may be involved in inter-kingdom communication (Bina and Bina. 2010, Campbell et al 2009, Park et al 2006.…”

“…Another study published by Campbell et al (2009), who determined that cFP inhibited quorum sensing activity through LuxR and also inhibited luminescence by reducing the number of active luciferases in A. fischeri. Although cFP was found to act on quorum sensing through LuxR, it did not compete with the primary autoinducers, the AHLs.…”

Design and Synthesis of S-ribosylhomocysteine Analogues

“…As the population of bacteria grows a positive feedback is started. 27 The AHL molecules are composed of fatty acyl chain linked to a lactonised homoserine through an amide bond. 28 Different structures of AHLs which modulate different regulatory proteins in different bacteria are presented in Table 2.…”