Gram-negative bacteria Pseudomonas aeruginosa is a wide-spread cause of different infections that is often found to be resistant to many different antibiotics. Detailed knowledge of these bacteria vital processes can facilitate the development of new effective antibiotics. RNA polymerase is one of the most common targets for antibiotics in bacteria. In this work optical trapping is used for the first time in order to study transcription initiation in model pathogenic organism P. aeruginosa. Three beads configuration have been chosen as an optimal scheme for trapping experiments in this study. The binding force between P. aeruginosa RNA polymerase and promoter region in DNA has been measured for the first time, during the assembly of promoter complex. The measured force was 13.1 ± 2.8 pN. These results could be used to search for new P. aeruginosa RNA polymerase inhibitors.