Increased exo gene transcription in strains overexpressing syrA. (A) Expression of exo::uidA transcriptional fusions. Plasmids harboring S. meliloti syrA were introduced into the exoY::pVO155, exoH::pVO155, and exoF::pVO155 mutants (which result in transcriptional fusions to the uidA gene). The strains were then grown to saturation and assayed for β-glucuronidase activity as described in Materials and Methods. Error bars represent standard deviations of experiments carried out in triplicate. Activity is in Miller units. Lane 1, exoY::pVO155/pTE3 (vector); lane 2, exoY::pVO155/pTE3::syrA; lane 3, exoH::pVO155/pTE3; lane 4, exoH::pVO155/pTE3::syrA; lane 5, exoF::pVO155/pTE3; lane 6, exoF::pVO155/pTE3::syrA. (B) Measurement of exoY expression by RT-PCR. Purified RNA was used as template for RT-PCR. Lane 1: cDNA prepared from wild type/pTE3, amplified with exoY-specific primers; lane 2, cDNA prepared from wild type/pTE3::syrA, amplified with exoY-specific primers; lane 3, cDNA prepared from wild type/pTE3, amplified with rpsF-specific primers; lane 4, cDNA prepared from wild type/pTE3::syrA, amplified with rpsF-specific primers.