462 ' hap 1 1 K 36 



encoded in a messenger RNA are produced of another? This spacing might be attrib- 

 in different quantities. uted to a nonsense triplet or to an intra- 

 The "operator*" gene might sometimes molecular base-pairing that results in seg- 

 prove to be nothing more than the nucleotide ments of a single-stranded messenger RNA 

 sequence which begins a scries of structural having a secondary structure (being "double- 

 genes in an operon. However, the question stranded" as in sRNA).' ; 

 remains: What is it in messenger RNA which 



serves as "spacer" — that is, what marks the .. c y . c c - „ r\ xx/ ,„„ oc . . „ , », , A/ 



r •■ See M. r. Singer, O. W. Jones, and M. W. 



termination of one polypeptide and the start Nirenberg ( 1963). 



SUMMARY AND CONCLUSIONS 



DNA genes can be classified as genes for structure or genes for function. A gene for 

 function serves as an operator controlling the expression of the structural genes which 

 are its linear neighbors. This whole complex of genes comprises an operon which is 

 probably a unit of transcription. Some operons are normally nonfunctional; they are 

 controlled by a regulator gene which produces a repressor which, in turn, represses the 

 operator and hence, the operon. Derepression of such operons can be accomplished by: 



1. An inducer (often the substrate for the first enzyme) which removes the repressor 



2. Mutation of the regulator gene (which changes the repressor) 



3. Mutation of the operator (which renders it insensitive to the repressor). 



Other operons are normally functional and are rendered nonfunctional by repressors. 

 Repression may result from mutations in regulator or operator genes which make an 

 operator gene susceptible to the repressor of a foreign regulator gene 



Regulator genes, inducers, and operons interact at both the transcriptional and trans- 

 lational levels in feedback systems of various types to regulate the production of proteins. 



REFERENCES 



Allen, J. M. (Ed.), The Molecular Control of Cellular Activity, New York: McGraw- 

 Hill. 1961. 



Ames, B. N., and Hartman, P. E., "The Histidine Operon," Cold Spring Harb. Sympos. 

 Quant. Biol., 28:349-361, 1964. 



Balis, M. E., Salser, J. S., and Elder, A., "A Suggested Role for Amino-acids in Deoxy- 

 ribonucleic Acid," Nature, London, 203:1170-1171. 1964. 



Cellular Regulatory Mechanisms, Cold Spring Harb. Sympos. Quant. Biol., 26, 1962. 



Gallant, J., and Spottswood, T., "Measurement of the Stability of the Repressor of 

 Alkaline Phosphatase Synthesis in Escherichia coli," Proc. Nat. Acad. Sci., U.S., 

 52:1591-1598, 1964. 



Garen, A., and Otsuji, N., "Isolation of a Protein Specified by a Regulator Gene," J. 

 Mol. Biol., 8:841-852, 1964, 



