Investigation into biological role of melD operon in Streptomyces

• Main Authors: Han-Yu Yang, 楊涵羽
• Other Authors: Carton W. Chen
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/49803151529589050393

碩士 === 國立陽明大學 === 生命科學暨基因體科學研究所 === 95 === About one third of Streptomyces species can produce extracellular tyrosinase, which catalyzes formation of melanin pigment (Mel+). These species harbor a melC operon consisting of melC1, which encodes a helper protein required for copper incorporation and secretion of the tyrosinase, and melC2, which encodes the tyrosinase. The chromosome of Streptomyces coelicolor encodes a homolog of melC operon (designated melD), although this species does not produce melanin (Mel-). Surprisingly, an ortholog of the melD operon also exists in the genomes of Streptomyces avermitilis and Streptomyces scabies, which also possess a melC operon and are Mel+. In all the three species, the melD is located centrally on the linear chromosomes, in contrast to the terminal location of the melC operon found in the Mel+ species. Fourteen melC and melD operons were collected from different Streptomyces species, and the two paralog groups are distinguished by (i) phylogenetic analysis, (ii) Ka/Ks ratio analysis, (iii) operon structure patterns (i.e., overlap between melD1 and melD2, but not between melC1 and melC2), and (iv) genetic stability (i.e., unstable melC vs. stable melD). No extracellular and intracellular tyrosinase activity was detected in both S. coelicolor, and the function of melD is not clear. To test the possible role of melD in protection against phenolic compounds (secreted as antimicrobial substances by plants in soil), a ΔmelD mutant of S. coelicolor was created and tested. The ΔmelD mutants showed increased sensitivity to pyrocatechol, but not to caffeic acid and 3,4-dihyroxybenzoic acid, implicating the protection against pyrocatechol damage by melD. Surprisingly, secreted MelC increases the sensitivity to pyrocatechol (by comparison between melC+ and ΔmelC strains). A model was proposed for the opposite effects of melC and melD on sensitivity to the phenolic compounds. Furthermore, His-tagged MelD2 was found at the membrane fraction of S. coelicolor with a reduced molecular weight (25 kDa vs. the expected 30 kDa), suggesting the translocation of MelD to the membrane through a proteolytic processing. In summary, melD differs distinctly from melC in the genetic structures, protein locations, substrate specificities, and mostly likely the biological role. While melD appears to be involved in protection against small phenolic compounds, the role of melC is less clear.