Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 239

The previous sections have reviewed how a gene is expressed in the cell, from the time mRNA is transcribed from the gene until the protein product of the gene reaches its final destination in or outside the cell and has its effect. In general, genes are expressed in the cell only when the product is needed by the cell and then only as much as is required to make the amount of product needed by the cell. This saves energy and prevents the products of different genes from interfering with each other. The process by which the output of genes is changed depending on the state of the cell is called the regulation of gene expression and can occur at any stage in the expression of the gene (see chapter 11).

Genes whose products regulate the expression of other genes are called regulatory genes. The product of a regulatory gene can either inhibit or stimulate the expression of a gene. If it inhibits expression, the regulation is negative; if it stimulates expression, the regulation is positive. Some regulatory gene products are both positive and negative regulators depending on the situation. The product of a regulatory gene can regulate the expression of only one other gene, or it can regulate the expression of many genes. The set of genes regulated by the same regulatory gene product is called a regulon. If a gene product regulates its own expression, it is said to be autoregulated. We discuss the molecular mechanisms of regulation of gene expression in much more detail in chapter 11, but in this chapter, we briefly review some basic concepts needed to understand the following chapters.

Figure 2.42 The sortase A pathway. (A) Typical sortase substrate. The protein is composed of an N-terminal signal peptide and a C-terminal cell wall-sorting signal (Cws). The Cws contains a conserved LPXTG motif followed by a hydrophobic stretch of amino acids and positively charged residues at the C terminus. (B) Model for the cell wall sortase A pathway in Staphylococcus aureus. (1) The full-length surface protein precursor is secreted through the cytoplasmic membrane via an N-terminal signal sequence. (2) A charged tail (+) at the C terminus of the protein may serve as a stop transfer signal. Following cleavage of this secretion signal, a sortase enzyme cleaves the protein between the threonine and glycine residues of the LPXTG motif, forming a thioacyl-enzyme intermediate to a specific cysteine in the sortase (3). It is then attached to the free amine of the five-glycine cross-bridge of lipid II (4) before transfer into the cell wall (5). The Pro-Gly-Ser-Thr region may help it through the thick cell wall so that it is expressed on the cell surface. PP is the site of MurNAc pentapeptide attachment to bactoprenyl in the membrane in lipid II. Modified from Connolly KM, Clubb RT, in Waksman G, Caparon M, Hultgren S (ed), Structural Biology of Bacterial Pathogenesis (ASM Press, Washington, DC, 2005).