Tryptophanase (tryptophan indole-lyase, Tnase, EC 4.1.99.1), a bacterial enzyme with no counterpart in eukaryotic cells, produces from L-tryptophan pyruvate, ammonia and indole. It was recently suggested that indole signaling plays an important role in the stable maintenance of multicopy plasmids. In addition, Tnase was shown to be capable of binding Rcd, a short RNA molecule involved in resolution of plasmid multimers. Binding of Rcd increases the affinity of Tnase for tryptophan, and it was proposed that indole is involved in bacteria multiplication and biofilm formation. Biofilm-associated bacteria may cause serious infections, and biofilm contamination of equipment and food, may result in expensive consequences. Thus, optimal and specific factors that interact with Tnase can be used as a tool to study the role of this multifunctional enzyme as well as antibacterial agents that may affect biofilm formation. Most known quasi-substrates inhibit Tnase at the mM range. In the present work, the mode of Tnase inhibition by the following compounds and the corresponding Ki values were: S-phenylbenzoquinone-L-tryptophan, uncompetitively, 101 microM; alpha-amino-2-(9,10-anthraquinone)-propanoic acid, noncompetitively, 174 microM; L-tryptophane-ethylester, competitively, 52 microM; N-acetyl-L-tryptophan, noncompetitively, 48 microM. S-phenylbenzoquinone-L-tryptophan and alpha-amino-2-(9,10-anthraquinone)-propanoic acid were newly synthesized.