Global consequences of phosphatidylcholine reduction in Bradyrhizobium japonicum

Phosphatidylcholine (PC) is the major phospholipid in eukaryotic membranes. In contrast, it is found in only a limited number of bacteria including members of the Rhizobiales. Here, PC is required for pathogenic and symbiotic plant-microbe interactions, as shown for Agrobacterium tumefaciens and Bradyrhizobium japonicum, respectively. Two different phospholipid N-methyltransferases, PmtA and PmtX1, convert phosphatidylethanolamine (PE) to PC by three consecutive methylation reactions in B. japonicum. PmtA mainly catalyzes the first methylation reaction converting PE to monomethyl PE, which then serves as substrate for PmtX1 performing the last two methylation reactions. Disruption of the pmtA gene results in a significantly reduced PC content causing a defect in symbiosis with the soybean host. A genome-wide survey for differentially expressed genes in the pmtA mutant with a custom-made Affymetrix gene chip revealed that PC reduction affects transcription of a strictly confined set of genes. Among the 11 up regulated genes were pmtX3 and pmtX4, which code for isoenzymes of PmtA. The expression of two typical two-component systems, a MarR-like regulator and two proteins of a RND-type (resistance nodulation cell division) efflux system were differentially expressed in the pmtA mutant. Our data suggests that a decrease in the PC content of B. japonicum membranes induces a rather specific transcriptional response involving three different transcriptional regulators all involved in the regulatory fine-tuning of a RND-type transport system.