Localization of the <i>reb</i> operon expression is inconsistent with that of the R-body production in the stem nodules formed by <i>Azorhizobium caulinodans</i> mutants having a deletion of <i>praR</i>

Abstract

From the infection pockets, infection threads guide the bacteria toward the nodule meristematic zone, wherein the bacteria are released into the host cells and surrounded by a plant-derived peribacteroid membrane (Tsien et al., 1983). The infected host cells are filled with differentiated bacteroids, and the infected area enlarges as the nodule matures (Dreyfus and Dommergues, 1981; Tsien et al., 1983). A. caulinodans can potentially kill the host plant cells by producing R-bodies (Matsuoka et al., 2017b). R-bodies are large proteinaceous ribbons that are coiled into cylindrical structures and were first observed in Caedibacter species, which are obligate bacterial endosymbionts of paramecia (Anderson et al., 1964). The paramecia that harbor the R-body-producing Caedibacter cells (i.e., killer paramecia) release the bacterial cells through their cytopyge. The paramecia without the endosymbiont (i.e., sensitive paramecia) ingest the released bacteria and die. A. caulinodans has a reb operon (locus tags on the genome, AZC_3781 to AZC_3788), containing four reb genes (AZC_3781, AZC_3782, AZC_3783, and AZC_3786), which is associated with R-body production (Heruth et al., 1994; Matsuoka et al., 2017b). In addition, three unknown genes (AZC_3784, AZC_3785, and AZC_3787), and a transcription factor gene, rebR (AZC_3788) are also contained in the reb operon. RebR binds to the promoter region of the reb operon and acts as an activator for the reb operon expression (Matsuoka et al., 2017b). The reb operon expression is usually suppressed by Localization of the reb operon expression is inconsistent with that of the R-body production in the stem nodules formed by Azorhizobium caulinodans mutants having a deletion of praR