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Glycerol inhibition of melanin biosynthesis in the environmental Aeromonas salmonicida 34melT

Abstract

The environmental strain Aeromonas salmonicida subsp. pectinolytica 34melT produces abundant melanin through the homogentisate pathway in several culture media, but unexpectedly not when grown in a medium containing glycerol. Using this observation as a starting point, this study investigated the underlying causes of the inhibition of melanin synthesis by glycerol, to shed light on factors that affect melanin production in this microorganism. The effect of different carbon sources on melanin formation was related to the degree of oxidation of their C atoms, as the more reduced substrates delayed melanization more than the more oxidized ones, although only glycerol completely abolished melanin production. Glyphosate, an inhibitor of aromatic amino acid synthesis, did not affect melanization, while bicyclopyrone, an inhibitor of 4-hydroxyphenylpyruvate dioxygenase (Hpd), the enzyme responsible for the synthesis of homogentisate, prevented melanin synthesis. These results showed that melanin production in 34melT depends on the degradation of aromatic amino acids from the growth medium and not on de novo aromatic amino acid synthesis. The presence of glycerol changed the secreted protein profile, but none of the proteins affected could be directly connected with melanin synthesis or transport. Transcription analysis of hpd, encoding the key enzyme for melanin synthesis, showed a clear inhibition caused by glycerol. The results obtained in this work indicate that a significant decrease in the transcription of hpd, together with a more reduced intracellular state, would lead to the abolishment of melanin synthesis observed. The effect of glycerol on melanization can thus be attributed to a combination of metabolic and regulatory effects.

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Acknowledgments

N.I.L. and M.J.P. are career investigators from CONICET. E.S.V. and D.E.E. hold doctoral fellowships from CONICET.

Funding

This work was partially supported by the University of Buenos Aires, CONICET, and Agencia Nacional de Promoción Científica y Tecnológica.

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M.E.P., N.I.L., and M.J.P. conceived the project and designed experiments. All authors carried out experiments. M.E.P., N.I.L., and M.J.P. wrote the manuscript. All authors reviewed the manuscript.

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Correspondence to Nancy I. López or M. Julia Pettinari.

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The authors declare that they have no conflict of interest.

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This article does not contain any studies with human participants or animals.

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Pavan, M.E., Venero, E.S., Egoburo, D.E. et al. Glycerol inhibition of melanin biosynthesis in the environmental Aeromonas salmonicida 34melT. Appl Microbiol Biotechnol 103, 1865–1876 (2019). https://doi.org/10.1007/s00253-018-9545-9

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  • DOI: https://doi.org/10.1007/s00253-018-9545-9

Keywords

  • Aeromonas salmonicida subsp. pectinolytica
  • Glycerol
  • Melanin inhibition
  • Homogentisate pathway
  • 4-Hydroxyphenylpyruvate dioxygenase
  • hpd expression