68
BIOCHEMICAL SOCIETY TRANSACTIONS
Hecht, N. B. & Woese, F. R. (1968) J. Bacteriol. 95, 986990 Kaltschmidt, E. & Wittmann, H. G. (1970) Anal. Biochem. 36, 401-412 Kaltschmidt, E. & Wittmann, H. G. (1972) Biochimie 54,167-175 Lewandowski, L. J. & Brownstein, B. L. (1969) J. Mol. Biol. 41, 277-290 Lindahl, L. (1975) J. Mol. Biol. 92,15-37 MacDonald, R. E., Turnock, G. & Forchhammer, J. (1967) Proc. Nutl. Acad. Sci. U.S.A. 57, 141-147
Mangiarotti, G., Apirion, D., Schlessinger, D. & Silengo, L. (1968) Biochemistry 7, 456-472 Nashimoto, H., Held, W., Kaltschmidt,E. & Nomura, M. (1971) J. Mol. Biol. 62,121-138 Nierhaus, K. H., Bordash, K. & Homann, H. E. (1973) J. Mol. Biol. 74, 587-597 Osawa, S., Otaka, E., Itoh, T. & Fukui, T. (1969) J. Mol. Biol. 40,321-351
Gene Expression and its Modification* SHERRY LEWIN Department of Postgraduate Molecular Biology, North East London Polytechnic, London El5 4LZ, U.K. Changes in gene expression arise from introduction, substitution or deletion of genes in the DNA genome. This has been expressed in the ‘Central Dogma’ (Crick, 1958, 1968,1970), which states that the direction of flow of genetic information is ($A
-
RNA
-
Protein
which signifies that, although DNA makes RNA, and RNA makes protein, some RNA species, using reverse transcriptase, are responsible for biosynthesis of DNA, but that protein is never responsible for RNA formation nor for DNA formation (for extensive references see Ycas, 1969; Wolstenholme & Knight, 1969; Lewin, B., 1974). The switching on and off of transcriptional activities in DNA cistrons follow dynamic processes which comprise (a) unmasking of specific genetic sites on the double helix, and/or (b) unwinding of specific genetic sites required by RNA polymerase. for expression of the genetic potential of DNA. For mechanisms to be biologically admissible, they must be stereochemically and energetically permissible. A guide to stereochemical permissibility can be obtained from the construction of correctly proportioned space-filling molecular models (e.g. Lewin, 1968, 1970, 1974a,b, 197546). Energetic permissibility can be evaluated from calculations based on graded interfacial-tension changes on specific sequences of hydrophobic-group de-adherances (Lewin, 1974a,b, 1975a,b). Unmasking of specific sites on the double helix may arise from hinged movements of nuclear proteins (Lewin, 1968, 1970, 1975b) or from contraction of these proteins as a result of random chain segments undergoing conformational change to the a-helix (since the area covered efficiently by an a-helix form is smaller than the corresponding random coil. Thermal unwinding of the double helix presents difficulties, since the T, values of DNA molecules and synthetic polydeoxyribonucleotide double helices are far too high to allow unwinding at the physiological temperature, p H values and ionic strength. The difficulties can be avoided if one invokes the assistance of compressor/ repressor proteins and extenderloperator proteins in the dynamic processes of unwinding and rewinding of the double helix (Lewin, S., 1974b, 19756). Unwinding of the double helix can be enforced as a result of conformational change, from a-helix to 8-conformation, in compressor/repressor proteins which are attached to DNA at respectiveendings of complementary genes (Lewin, S., 1974b, 19756).Extension of the protein, after interaction with its complementary extender/operator protein to form a 8-conformation, results in corresponding enforced extension of the DNA in which the double helix is unwound (see, e.g., Lewin, S., 1974~).The reverse process results in rewinding the DNA into the double helix. * Not presented at the Meeting, owing to the death of Dr. Lewin. 1976
560th MEETING, OXFORD
69
On these lines the activity of structural/regulatory proteins in pretranscriptional unwinding of double-helical cistrons is visualized to involve nuclear genes in different types of mutation, which may be termed gene mutation, protein mutation and environmental mutation. A gene mutation may be defined as a change in genetic expression arising directly from a change in the genetic sequence of DNA caused by introduction, substitution or deletion of one or more genes. In a protein mutation there is no change in the DNA sequence, but there is a change in the expression of the genetic potential of the DNA after changes in the amino acid sequence of the compressorlrepressor protein, as a result of which the particular protein no longer adheres to the entire original length or any of the DNA. Here the change in regulatory/structural proteins would originate at a post-RNA translational stage, i.e. at a ribosomal protein-synthesis stage. In an environmental mutation there is again no change in the genetic sequence of DNA, nor in the structural/regulatory proteins, but there is a change in the expression of the genetic potential of DNA after changes in the interfacial tension of the environment within the nucleus, and therefore in hydrophobic adherences between the DNA and the compressor/repressor protein, or changes in water availability and therefore in the apparent strength of the hydrogen bonds, as a result of which the protein no longer adheres to the whole or any part of the originally complementary cistron of DNA. Additional changes in genetic expression, which may be classified in the last two categories, may arise from corresponding influences in the extender/operator proteins with respect to their interactions with their respective complementary compressor/repressor proteins. On this basis the 'Central Dogma' would be more representative when presented as
-
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BIOCHEMICAL SOCIETY TRANSACTIONS
Hecht, N. B. & Woese, F. R. (1968) J. Bacteriol. 95, 986990 Kaltschmidt, E. & Wittmann, H. G. (1970) Anal. Biochem. 36, 401-412 Kaltschmidt, E. & Wittmann, H. G. (1972) Biochimie 54,167-175 Lewandowski, L. J. & Brownstein, B. L. (1969) J. Mol. Biol. 41, 277-290 Lindahl, L. (1975) J. Mol. Biol. 92,15-37 MacDonald, R. E., Turnock, G. & Forchhammer, J. (1967) Proc. Nutl. Acad. Sci. U.S.A. 57, 141-147
Mangiarotti, G., Apirion, D., Schlessinger, D. & Silengo, L. (1968) Biochemistry 7, 456-472 Nashimoto, H., Held, W., Kaltschmidt,E. & Nomura, M. (1971) J. Mol. Biol. 62,121-138 Nierhaus, K. H., Bordash, K. & Homann, H. E. (1973) J. Mol. Biol. 74, 587-597 Osawa, S., Otaka, E., Itoh, T. & Fukui, T. (1969) J. Mol. Biol. 40,321-351
Gene Expression and its Modification* SHERRY LEWIN Department of Postgraduate Molecular Biology, North East London Polytechnic, London El5 4LZ, U.K. Changes in gene expression arise from introduction, substitution or deletion of genes in the DNA genome. This has been expressed in the ‘Central Dogma’ (Crick, 1958, 1968,1970), which states that the direction of flow of genetic information is ($A
-
RNA
-
Protein
which signifies that, although DNA makes RNA, and RNA makes protein, some RNA species, using reverse transcriptase, are responsible for biosynthesis of DNA, but that protein is never responsible for RNA formation nor for DNA formation (for extensive references see Ycas, 1969; Wolstenholme & Knight, 1969; Lewin, B., 1974). The switching on and off of transcriptional activities in DNA cistrons follow dynamic processes which comprise (a) unmasking of specific genetic sites on the double helix, and/or (b) unwinding of specific genetic sites required by RNA polymerase. for expression of the genetic potential of DNA. For mechanisms to be biologically admissible, they must be stereochemically and energetically permissible. A guide to stereochemical permissibility can be obtained from the construction of correctly proportioned space-filling molecular models (e.g. Lewin, 1968, 1970, 1974a,b, 197546). Energetic permissibility can be evaluated from calculations based on graded interfacial-tension changes on specific sequences of hydrophobic-group de-adherances (Lewin, 1974a,b, 1975a,b). Unmasking of specific sites on the double helix may arise from hinged movements of nuclear proteins (Lewin, 1968, 1970, 1975b) or from contraction of these proteins as a result of random chain segments undergoing conformational change to the a-helix (since the area covered efficiently by an a-helix form is smaller than the corresponding random coil. Thermal unwinding of the double helix presents difficulties, since the T, values of DNA molecules and synthetic polydeoxyribonucleotide double helices are far too high to allow unwinding at the physiological temperature, p H values and ionic strength. The difficulties can be avoided if one invokes the assistance of compressor/ repressor proteins and extenderloperator proteins in the dynamic processes of unwinding and rewinding of the double helix (Lewin, S., 1974b, 19756). Unwinding of the double helix can be enforced as a result of conformational change, from a-helix to 8-conformation, in compressor/repressor proteins which are attached to DNA at respectiveendings of complementary genes (Lewin, S., 1974b, 19756).Extension of the protein, after interaction with its complementary extender/operator protein to form a 8-conformation, results in corresponding enforced extension of the DNA in which the double helix is unwound (see, e.g., Lewin, S., 1974~).The reverse process results in rewinding the DNA into the double helix. * Not presented at the Meeting, owing to the death of Dr. Lewin. 1976
560th MEETING, OXFORD
69
On these lines the activity of structural/regulatory proteins in pretranscriptional unwinding of double-helical cistrons is visualized to involve nuclear genes in different types of mutation, which may be termed gene mutation, protein mutation and environmental mutation. A gene mutation may be defined as a change in genetic expression arising directly from a change in the genetic sequence of DNA caused by introduction, substitution or deletion of one or more genes. In a protein mutation there is no change in the DNA sequence, but there is a change in the expression of the genetic potential of the DNA after changes in the amino acid sequence of the compressorlrepressor protein, as a result of which the particular protein no longer adheres to the entire original length or any of the DNA. Here the change in regulatory/structural proteins would originate at a post-RNA translational stage, i.e. at a ribosomal protein-synthesis stage. In an environmental mutation there is again no change in the genetic sequence of DNA, nor in the structural/regulatory proteins, but there is a change in the expression of the genetic potential of DNA after changes in the interfacial tension of the environment within the nucleus, and therefore in hydrophobic adherences between the DNA and the compressor/repressor protein, or changes in water availability and therefore in the apparent strength of the hydrogen bonds, as a result of which the protein no longer adheres to the whole or any part of the originally complementary cistron of DNA. Additional changes in genetic expression, which may be classified in the last two categories, may arise from corresponding influences in the extender/operator proteins with respect to their interactions with their respective complementary compressor/repressor proteins. On this basis the 'Central Dogma' would be more representative when presented as
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