| Popis: |
This chapter recognizes John Roth's contributions to the genetic study of microbial metabolism by summarizing results in an area that the authors have explored over the years. Wild-type Escherichia coli can make biotin (vitamin H) from an inorganic medium with glucose as the sole carbon source. In the course of studies on prophage insertion, the authors isolated both specialized transducing phages and deletions that included the mutant site, which facilitated asking how many bio genes were clustered there, what they did, and how they were regulated. However, the biotin operon intrigued the authors because the product is required in such minute amounts, roughly 1,000 molecules per cell. The repressor protein BirA is encoded by a gene distant from the bio operon. BirA also functions as a biotin ligase. The biotin operon is transcribed bidirectionally, from overlapping promoters controlled by the same operator. The authors discuss at the outset why they thought biotin biosynthesis and its control might be interesting. Biotin-d-sulfoxide (BDS) is reduced by a specific reductase (BisC) and a molybdopterin cofactor (functional also in the reduction of nitrate and dimethyl sulfoxide), whose biosynthesis requires at least 15 genes. If BDS is added to wild-type cells at high concentration, the biotin operon is repressed. The authors turned the regulatory system around and used it as a possible selection for mutations that knock out BDS reduction. |
