Down-regulation of the Rhizobium ntr system in the determinate nodule of Phaseolus vulgaris identifies a specific developmental zone

The Rhizobium NtrBC two-component system, although essential for the expression of genes involved in ammonium assimilation, is not required for nodule development and efficiency, Moreover, nitrogen assimilation was previously found to be absent in nitrogen-fixing bacteroids. We speculate that this switch-off in nitrogen assimilation may be mediated by the inactivation (dephosphorylation) of the NtrC protein or by its absence, We find that transcription of the Rhizobium etli ORF1-ntrBC operon is switched off during bacteroid differentiation in coincidence with the arrest of bacterial division occurring in the nodule, Free-living R, etli shows a similar behavior, since the ORF1-ntrBC promoter is active only in growing bacteria, The NtrC protein is expressed only in the young bacteroids located in the peripheral part of the nodule of Phaseolus vulgaris and this identifies the presence of a previously unrecognized zone of development in determinate nodules. The NtrC protein disappears from bacteroids within one cell layer, thus indicating protein turnover coincident with the arrest in bacterial division, In indeterminate type Vicia hirsuta nodules down-regulation of the ORF1-ntrBC promoter occurs when bacteroids stop dividing and before the developmental stage in which they express the nifH gene. We discuss the possibility that invagination in the peribacteroidal membrane of the bacteroids may inhibit their multiplication and, as a consequence, cause down-regulation of a set of genes as part of a developmental mechanism required to reduce bacteroid metabolic activities.