Sinorhizobium meliloti 1021 responds to external biotin signals from alfalfa plants through the bioS regulatory locus. Immunogold labeling and electron microscopy revealed that the BioS protein is located within the S. meliloti cytoplasm. Under biotin-limiting conditions the S. meliloti cell lumen was filled with polyhydroxybutyrate (PHB) granules suggesting that either PHB synthesis or degradation are influenced by biotin. To test this hypothesis a 3-hydroxybutyrate-dehydrogenase-lacZ (bdhA-lacZ) fusion was mobilized into S. meliloti. beta-galactosidase tests revealed an overall 3.6-5.2-fold higher bdhA transcription in the presence of added biotin. Comparison of the bdhA and the bioS promoter regions identified several common motifs.