Systematic analysis of theMyxococcus xanthusdevelopmental gene regulatory network supports posttranslational regulation of FruA by C-signaling

Abstract

25Upon starvation Myxococcus xanthus undergoes multicellular development. Rod-shaped cells move 26 into mounds in which some cells differentiate into spores. Cells begin committing to sporulation at 24-27 30 h poststarvation, but the mechanisms governing commitment are unknown. FruA and MrpC are 28 transcription factors that are necessary for commitment. They bind cooperatively to promoter regions 29 and activate developmental gene transcription, including that of the dev operon. Leading up to and 30 during the commitment period, dev mRNA increased in wild type, but not in a mutant defective in C-31 signaling, a short-range signaling interaction between cells that is also necessary for commitment. The 32 C-signaling mutant exhibited ~20-fold less dev mRNA than wild type at 30 h poststarvation, despite a 33 similar level of MrpC and only twofold less FruA. Boosting the FruA level twofold in the C-signaling 34 mutant had little effect on the dev mRNA level, and dev mRNA was not less stable in the C-signaling 35 mutant. Neither did high cooperativity of MrpC and FruA binding upstream of the dev promoter 36 explain the data. Rather, our systematic experimental and computational analyses support a model in 37 which C-signaling activates FruA at least ninefold posttranslationally in order to commit a cell to spore 38 formation. 39 40

Ultrasensitive Response of Developing Myxococcus xanthus to the Addition of Nutrient Medium Correlates with the Level of MrpC

“…A rising level of FruA* produced in response to C-signaling during development would not only promote mound formation, it would trigger gene expression and lead to sporulation (56)(57)(58). FruA* and MrpC have been proposed to bind cooperatively to activate the transcription of genes that commit rod-shaped cells to form spores (5,28,35) (Fig. 1).…”

“…Alternatively, or in addition, MrpC itself could be a nutrient-sensing autorepressor. In two recent studies, MrpC was shown to negatively autoregulate (5,62), and it may do so by competing with MrpB-P for overlapping binding sites in the mrpC promoter region (62). It is also possible that CTTYE addition destabilizes mrpC transcripts.…”

“…An inducible system capable of higher expression during development will need to be engineered. The mrpC promoter may be a good starting point, since the mrpC transcript is highly abundant, about 10% of the 16S rRNA level in cells at 24 h PS (5).…”

“…The differentiation of rod-shaped cells inside mounds into to move into alignment (36)(37)(38), as they do during mound formation (39)(40)(41). C-signaling appears to activate FruA (5,20), but the mechanism of activation is unknown. FruA* may cause methylation of the FrzCD component (42,43) of the Frz chemosensory system that controls the gliding movements of cells (44), since C-signaling alters cell movements to promote mound formation (45,46).…”

“…Under these conditions, wild-type M. xanthus strain DK1622 forms mounds between 15 and 18 h poststarvation (PS) (35). Sonication-resistant spores form by 27 h PS, and the number of spores increases greatly by 36 to 48 h PS (5,35). The critical period of commitment to sporulation was found to be between 24 and 30 h PS, since adding nutrient medium before or at 24 h blocked sporulation, whereas adding nutrient medium at 30 h or later did not block sporulation (35).…”