Regulation of the Myxococcus xanthus C-Signal-Dependent Ω4400 Promoter by the Essential Developmental Protein FruA

The bacterium Myxococcus xanthus employs extracellular signals to coordinate aggregation and sporulation during multicellular development. Extracellular, contact-dependent signaling that involves the CsgA protein (called C-signaling) activates FruA, a putative response regulator that governs a branched signaling pathway inside cells. One branch regulates cell movement, leading to aggregation. The other branch regulates gene expression, leading to sporulation. C-signaling is required for full expression of most genes induced after 6 h into development, including the gene identified by Tn5 lac insertion Omega4400. To determine if FruA is a direct regulator of Omega4400 transcription, a combination of in vivo and in vitro experiments was performed. Omega4400 expression was abolished in a fruA mutant. The DNA-binding domain of FruA bound specifically to DNA upstream of the promoter -35 region in vitro. Mutations between bp -86 and -77 greatly reduced binding. One of these mutations had been shown previously to reduce Omega4400 expression in vivo and make it independent of C-signaling. For the first time, chromatin immunoprecipitation (ChIP) experiments were performed on M. xanthus. The ChIP experiments demonstrated that FruA is associated with the Omega4400 promoter region late in development, even in the absence of C-signaling. Based on these results, we propose that FruA directly activates Omega4400 transcription to a moderate level prior to C-signaling and, in response to C-signaling, binds near bp -80 and activates transcription to a higher level. Also, the highly localized effects of mutations between bp -86 and -77 on DNA binding in vitro, together with recently published footprints, allow us to predict a consensus binding site of GTCG/CGA/G for the FruA DNA-binding domain.