Differential gene activity definition

For the purposes of this review, it is therefore necessary to provide a working definition of the term differentiation factor. I will limit the discussion to those substances which, in addition to their potential functions elsewhere in the body, play a definitive role in the differentiation of the nervous system. In this context, the term will be used to denote only molecules with the ability to induce the novel expression of genes rather than those which merely amplify existing expression. In particular, this review will focus on substances involved in the activation of those genes encoding neurotransmitter biosynthetic enzymes since their expression is a requisite first step in the specification of biochemical phenotype. This restrictive definition excludes many molecules which have historically been considered differentiation factors (i.e. nerve growth factor) since they do not initiate, but merely modulate, expression of transmitter-associated genes. 

Also of interest, P5C reductase from various tissue or cellular sources are differentially sensitive to inhibition by proline (99,117), NADP (86, 117), and adenine nucleotides (86). The enzyme from cultured fibroblasts is sensitive to inhibition by proline and its inhibition of the NADH-mediated reaction (K, = 2 X 10 M) is much greater than of the NADPH-mediated reaction (K = 2 X 10 M). By contrast, the hepatic and erythrocyte enzymes are insensitive to proline but very sensitive to inhibition by NADP+ (H7). Adenine nucleotides also inhibit the hepatic enzyme (86) but not the erythrocyte enzyme. These differential patterns of inhibition suggest that there are isozymes of P5C reductase found in various tissues, an interpretation further supported by the relative deficiency of the NADH-mediated activity in a leukemia cell line (55). Whether these putative isozymes are products of different genes or are interconvertible forms is an intriguing question which awaits definitive studies on enzyme prepared from different sources. 

The material dealing with functional activity of chromosomes and changes in the spectrum of proteins and nucleic acids during morphogenesis, described in the previous chapters, have revealed the pattern of purposive differentiated activity of the genes in ontogenesis and has indicated the external manifestation of this activity in the form of synthesis of essential products at a definite time and in a definite place. 

The second type controls the constancy of maintenance of specialization (differentiation) through a mechanism of permanent or, more exactly, prolonged repression of definite groups of genes or activation of other definite groups of genes. 

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