Metals, also responsive elements

Hormone response elements (for steroids, T3, retinoic acid, peptides, etc) act as—or in conjunction with— enhancers or silencers (Chapter 43). Other processes that enhance or silence gene expression—such as the response to heat shock, heavy metals (Cd and Zn +), and some toxic chemicals (eg, dioxin)—are mediated through specific regulatory elements. Tissue-specific expression of genes (eg, the albumin gene in liver, the hemoglobin gene in reticulocytes) is also mediated by specific DNA sequences. 

The evolution of MRSA strains is not fully understood, but the same mechanisms of mutation and gene transfer that exist in other species provide a likely reason. The emergence of gentamicin resistance plasmids illustrates the evolutionary potential of translocatable elements [186], MRSA strains which are also resistant to this aminoglycoside antibiotic are referred to as MGRSA. This evolutionary progression is also responsible for the formation of the -lactamase-heavy metal resistant plasmids [250]. Some MRSA isolates are penicillin-resistant by virtue of the enzyme /J-lactamase, which pre-dates the use of /8-lactams [251], However, the spread of the phenotype has probably arisen as a result of selection caused by the widespread usage of methicillin in hospitals. 

Beside the beneficial effect of the addition alloying metallic elements that contribute to the increased corrosion resistance, the amorphous structure itself is also responsible for the very low corrosion. For example, crystalline alloys with the same composition exhibit high rates of dissolution. The chemically homogeneous, single-phase nature of amorphous alloys is believed to account for their corrosion resistance (8, 100, 101). This also allows for the formation of a uniform, protective film on the surface of amorphous alloy electrodes. 

Most of the elements in the Periodic Table (in the pnre state) are metallic in natnre. Aluminnm, copper, and iron are examples. The metalhc bond involves a mobile gas of electrons. This gas of negatively charged electrons binds together the positively charged atomic cores. The electron gas is also responsible for the electrical condnctivities and optical absorption that are characteristic of metals. 

Nutrient control. In addition to the sterols and fat-soluble vitamins, other dietary constituents are also recognized by transcriptional activators or repres-sors. ° These include glucose, - amino acids, phosphate ions, and various metal ions. Best known among the latter is iron. The mammalian iron response element (IRE) is a hairpin loop RNA structure, which like the bacterial attenuator system (Fig. 

---