Genes synthases

The biosynthesis of maduropeptin has not been studied in detail, but an iterative type I polyketide synthase gene predicted to be responsible for forming the enediyne core structure has been identified [188]. 

Overproduction of the chromosomal genes for the dihydrofolate reductase (DHFR) and the dihydroptero-ate synthase (DHPS) leads to a decreased susceptibility to trimethoprim and sulfamethoxazol, respectively. This is thought to be the effect of titrating out the antibiotics. However, clinically significant resistance is always associated with amino acid changes within the target enzymes leading to a decreased affinity of the antibiotics. 

Phospholipase D is widely distributed in bacteria, fungi, plants and animals, and is present in almost all mammalian cells [3]. In mammals, it occurs as alternatively spliced products of two genes (PLD1 andPLD2) (Fig. 3). Most mammalian cells express different levels of both isoforms. Both PLD1 and PLD2 have four conserved sequences (I-IV), and sequences I and IV contain the HXKX4D (HKD) motif that is characteristic of the PLD superfamily, which includes bacterial endonucleases, phospholipid synthases, viral envelope... 

In bacteria and plants, the individual enzymes of the fatty acid synthase system are separate, and the acyl radicals are found in combination with a protein called the acyl carrier protein (ACP). However, in yeast, mammals, and birds, the synthase system is a multienzyme polypeptide complex that incorporates ACP, which takes over the role of CoA. It contains the vitamin pantothenic acid in the form of 4 -phosphopan-tetheine (Figure 45-18). The use of one multienzyme functional unit has the advantages of achieving the effect of compartmentalization of the process within the cell without the erection of permeability barriers, and synthesis of all enzymes in the complex is coordinated since it is encoded by a single gene. 

Since the discovery of the role of NO as a vasodilator, there has been intense experimental interest in this substance. It has turned out to have a variety of physiologic roles, involving virtually every tissue of the body (Table 49—9). Three major isoforms of NO synthase have been identified, each of which has been cloned, and the chromosomal locations of their genes in hu-... 

Table 49-8. Summary of the nomenclature of the NO synthases and of the effects of knockout of their genes in mice. ...

Clearly, the control of gene expression at the transcriptional level is a key regulatory mechanism controlling carotenogenesis in vivo. However, post-transcriptional regulation of carotenoid biosynthesis enzymes has been found in chromoplasts of the daffodil. The enzymes phytoene synthase (PSY) and phytoene desaturase (PDS) are inactive in the soluble fraction of the plastid, but are active when membrane-bound (Al-Babili et al, 1996 Schledz et al, 1996). The presence of inactive proteins indicates that a post-translational regulation mechanism is present and is linked to the redox state of the membrane-bound electron acceptors. In addition, substrate specificity of the P- and e-lycopene cyclases may control the proportions of the p, P and P, e carotenoids in plants (Cunningham et al, 1996). 

FRAY R, WALLACE A, FRASER P D, VALERO D, HEDDEN P, BRAMLEY P M and GRIERSON D (1995) Constitutive expression of a fruit phytoene synthase gene in transgenic tomatoes causes dwarfism by redirecting metabolites from the gibberellin pathway . Plant J, 8, 696-701. 

MATH s K, HEARST J E and pouLTER c D (1992) The crtE gene mErwinia herbicola encodes geranylgeranyl diphosphate synthase , Proc Natl Acad Sci, 89, 6761-4. 

Expression of a tomato cDNA coding for phytoene synthase in Escherichia coli, ph)doene formation in vivo and in vitro, and functional analysis of the varions trimcated gene prodncts , J Biochem (Tokyo), 116, 980-85. 

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