The tonB1 and tonB2 clusters are transcribed as separate operons. RT-PCR was performed with a downstream primer for the RT reaction in exbD1 and in tonB2 for the tonB1 and tonB2 clusters, respectively. In the subsequent PCR, specific primer sets for each gene were used to identify their presence in the same cDNA sample. (A) Lanes: 1, tonB1 RT-PCR product; 2, exbB1 RT-PCR product; 3, exbD1 RT-PCR product; 4, control with no reverse transcriptase; 5, H2O control; 6, molecular weight marker. (B) Lanes: 1, exbB2 RT-PCR product; 2, exbD2 RT-PCR product; 3, tonB2 RT-PCR product; 4, control with no reverse transcriptase; 5, H2O control; 6, molecular weight marker. (C) Lanes: 1, tolR RT-PCR product; 2, control with no reverse transcriptase; 3, molecular weight marker.