Eucaryotes

Fermentation. The commercial P-lactam antibiotics which act as starting material for all of the cephalosporins ate produced by submerged fermentation. The organisms used for the commercial production of the penicillins and cephalosporins ate mutants of PenicU/in chTysogenum and Cephalosporium acremonium respectively (3,153,154). Both ate tme fungi (eucaryotes). In contrast, the cephamycins ate produced by certain species of procaryotic Streptomyces including Streptomyces clavuligerus and Streptomyces lipmanii (21,103). 

Vimses are one of the smallest biological entities (except viroids and prions) that carry all the iaformation necessary for thek own reproduction. They are unique, differing from procaryotes and eucaryotes ia that they carry only one type of nucleic acid as genetic material, which can be transported by the vims from one cell to another. Vimses are composed of a shell of proteki enclosing a core of nucleic acid, either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA), that codes for vkal reproduction. The outer shell serves as a protective coat to keep the nucleic acid kitact and safe from enzymatic destmction. In addition to thek proteki coat, some vimses contain an outer covering known as an outer envelope. This outer envelope consists of a Hpid or polysaccharide material. 

CH2SH + 1/2 O2 -CH2-S-S-CH2 + H2O This reaction requires an oxidative environment, and such disulfide bridges are usually not found in intracellular proteins, which spend their lifetime in an essentially reductive environment. Disulfide bridges do, however, occur quite frequently among extracellular proteins that are secreted from cells, and in eucaryotes, formation of these bridges occurs within the lumen of the endoplasmic reticulum, the first compartment of the secretory pathway. 

The specific arrangement of two a helices joined by a loop region in lambda Cro and repressor, as well as in CAP, constitute the helix-turn-helix DNA-binding motif (Figure 8.8), which also occurs in some eucaryotic transcription factors as discussed in Chapter 9. The orientation of the two helices and... 

Figure 9.1 The transcriptional elements of a eucaryotic structural gene extend over a large region of DNA. The regulatory sequences can be divided into three main regions (1) the basal promoter elements such as the TATA box, (2) the promoter proximal elements close to the initiation site, and (3) distal enhancer elements far from the initiation site.

The general transcription factor TFllD is believed to be the key link between specific transcription factors and the general preinitiation complex. However, the purification and molecular characterization of TFllD from higher eucaryotes have been hampered by its instability and heterogeneity. All preparations of TFllD contain the TATA box-binding protein in combination with a variety of different proteins called TBP-associated factors, TAFs. When the preinitiation complex has been assembled, strand separation of the DNA duplex occurs at the transcription start site, and RNA polymerase II is released from the promoter to initiate transcription. However, TFIID can remain bound to the core promoter and support rapid reinitiation of transcription by recruiting another molecule of RNA polymerase. 

The two homologous repeats, each of 88 amino acids, at both ends of the TBP DNA-binding domain form two stmcturally very similar motifs. The two motifs each comprise an antiparallel p sheet of five strands and two helices (Figure 9.4). These two motifs are joined together by a short loop to make a 10-stranded p sheet which forms a saddle-shaped molecule. The loops that connect p strands 2 and 3 of each motif can be visualized as the stirmps of this molecular saddle. The underside of the saddle forms a concave surface built up by the central eight strands of the p sheet (see Figure 9.4a). Side chains from this side of the P sheet, as well as residues from the stirrups, form the DNA-binding site. No a helices are involved in the interaction area, in contrast to the situation in most other eucaryotic transcription factors (see below). 

Homeodomain proteins are involved in the development of many eucaryotic organisms... 

Eucaryotes have many more genes and a broader range of specific transcription factors than procaryotes and gene expression is regulated by using sets of these factors in a combinatorial way. Eucaryotes have found several different solutions to the problem of producing a three-dimensional scaffold that allows a protein to interact specifically with DNA. In the next chapter we shall discuss some of the solutions that have no counterpart in procaryotes. However, the procaryotic helix-turn-helix solution to this problem (see Chapter 8) is also exploited in eucaryotes, in homeodomain proteins and some other families of transcription factors. 

Goodrich, J.A., Tjian, R. TBP TAF complexes selectivity factors for eucaryotic transcription. Curr. Opin. Cell Biol. 6 403-409, 1994. 

The DNA-binding motifs discussed in this and the preceding two chapters are those most frequently found in procaryotes and eucaryotes. However, other motifs are known, for example the p sheet motif of the met repressor in Escherichia coli which binds to the major groove of DNA. No doubt others remain to be discovered. 

Leucine zippers provide dimerization interactions for some eucaryotic transcription factors... 

The ability of the leucine zipper proteins to form heterodimers greatly expands the repertoire of DNA-binding specificities that these proteins can display. As illustrated in Figure 10.19, for example, three distinct DNA-binding specificities could, in principle, be generated from two types of monomer, while six could be created from three types of monomer and so on. This is an example of combinatorial control, in which combinations of proteins, rather than individual proteins, control a cellular process. It is one of the most important mechanisms used by eucaryotic cells to control gene expression. 

TFIIA and TFIIB bind to both TBP and DNA Flomeodomain proteins are involved in the development of many eucaryotic organisms Monomers of homeodomain proteins bind to DNA through a heltx-turn-helix motif In vivo specificity of homeodomain... 

The pharmacological receptor of cardiac glycosides is the sarcolemmal Na+/K+-ATPase expressed on most eucaryotic membranes. It was characterised biochemically in 1957 by J. Skou, who was awarded with the Nobel Prize in chemistry in 1997. The sodium... 

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