The aim of this work was to investigate the regulation mechanism of gene expression of the hbpRCAD-cluster in Pseudomonas azelaica HBPI. This cluster allowed the bacterium to use 2-hydroxybiphenyl and 2,2'-dihydroxybiphenyl as a sole carbon- and energy-source. The first gene of the cluster, hbpR, encodes a putative regulatory protein called HbpR. Following and in divergent orientation are three structural genes encoding the enzymes of the 2-HBP-degradation pathway. In between hbpR and hbpC was a region carrying in one direction a promoter for the hbpR gene and in the other direction a promoter for the hbpC gene. Between hbpC and hbpA, no promoter sequence was found. However, in front of the hbpD-gene also a promoter was found. Transcription activation of the hbpC- and hbpD- promoters occurred in the presence of 2-HBP and was mediated by the regulatory protein HbpR.The gene hbpR appeared to be expressed constitutively. The essential role of HbpR in regulating expression of the hbpCD and hbpA-genes was determined from constructed hbpR knockout mutants of P. azelaica HBP I and from heterologous expression in Escherichia coli. The hbpR gene in P. azelaica was disrupted by homologous recombination. Then we compared the bioluminescence of cellB with functional HbpR protein with that of cells without functional HbpR protein. The results showed that HbpR is important to start transcription of hbpC and hbpD. The transcript of hbpA is supposed to be on a mRNA together with the transcript of hbpC. The expression of hbpR seemed to be constitutively and occured also in cells without a functional HbpR protein. To substantiate the role of HbpR and the possible promoter regions further, primer extension experiments were carried out. Clear transcripts for hbpC and hbpD were found in RNA isolated from P. azelaica induced with 2-hydroxybiphenyl, but not with succinate. No clear transcripts were detected from hbpR or hbpA. Several aromatic compounds were tested for their ability to activate HbpR. Four were found. Those were 2-hydroxybipheny, 2,2'-dihydroxybiphenyl, 2-aminobiphenyl and 2-hydroxydiphenylmethane.