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Regulation of polyphenol oxidase activities and melanin synthesis in Marinomonas mediterranea: identification of ppoS, a gene encoding a sensor histidine kinase^a

Department of Genetics and Microbiology, Faculty of Biology¹ and Department of Biochemistry and Molecular Biology B², University of Murcia, 30100 Murcia, Spain

Author for correspondence: Antonio Sanchez-Amat. Tel: +34 968 364955. Fax: +34 968 363963. e-mail: antonio{at}um.es

Marinomonas mediterranea is a melanogenic marine bacterium that expresses two different polyphenol oxidases. One of them is a multipotent laccase able to oxidize a wide range of substrates. The second enzyme is an SDS-activated tyrosinase. Using transposon mutagenesis, a mutant affected in the regulation of both polyphenol oxidase activities and melanogenesis has been isolated. The sequencing of the gene disrupted by the mini-Tn10 transposon in this mutant indicates that it encodes a hybrid sensor kinase. This sensor kinase shows three phosphorylated conserved domains: the transmitter domain containing a histidine site typical of sensor kinases, a receiver domain with an aspartate residue and an additional phosphotransferase domain with a second conserved histidine. This structural organization is characteristic of kinases participating in a phosphorelay system. Northern blot and lacZ operon fusions indicate that the multipotent laccase activity is regulated not only by PpoS but also by growth phase at the transcriptional level. These results suggest that PPO activities and melanin synthesis play a role in the adaptive response of M. mediterranea to stressful environmental conditions.

Keywords: Marinomonas mediterranea, marine bacterium, two-component histidine kinase, laccase, tyrosinase

Abbreviations: Amp, ampicillin; DMP, dimethoxyphenol; DMPO, DMP oxidase; DO, dopa oxidase; Gm, gentamicin; Km, kanamycin; L-dopa, 3,4-dihydroxyphenylalanine; PPO, polyphenol oxidase; RACE, rapid amplification of cDNA ends; Rif, rifampicin; SO, syringaldazine oxidase; TH, tyrosine hydroxylase

^a The GenBank accession number for the sequence reported in this paper is AF398464.

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