actinomycete any prokaryote placed in the phy-
lum actinobacteria (see Appendix A). Actinomycetes
belonging to the genus Streptomyces produce a large
number of the antibiotics, of which actinomycin D
(q.v.) is an example.
actinomycin D an antibiotic produced by Strepto-myces chrysomallus that prevents the transcription of messenger RNA. See RNA polymerase.
activated macrophage a macrophage that hasbeen stimulated (usually by a lymphokine) to en-large, to increase its enzymatic content, and to in-crease its nonspecific phagocytic activity.
activating enzyme an enzyme that catalyzes a re-action involving ATP and a specific amino acid. The product is an activated complex that subsequently reacts with a specific transfer RNA.
activation analysis a method of extremely sensi-tive analysis based on the detection of characteristic radionuclides produced by neutron activation.
activation energy the energy required for a chem-ical reaction to proceed. Enzymes (q.v.) combine transiently with a reactant to produce a new com-plex that has a lower activation energy.
Under these circumstances the reaction can take place at the pre-vailing temperature of the biological system. Once the product is formed, the enzyme is released un-changed.
activator a molecule that converts a repressor into a stimulator of operon transcription; e.g., the repres-sor of a bacterial arabinose operon becomes an acti- vator when combined with the substrate.
Activator-Dissociation system a pair of interacting genetic elements in maize discovered and analyzed by Barbara McClintock. Ac is an autonomous ele- ment that is inherently unstable. It has the ability to excise itself from one chromosomal site and to transpose to another. Ac is detected by its activation of Ds. Ds is nonautonomous and is not capable of ex- cision or transposition by itself. Ac need not be adja- cent to Ds or even on the same chromosome in or- der to activate Ds. When Ds is so activated, it can alter the level of expression of neighboring genes, the structure of the gene product, or the time of de- velopment when the gene expresses itself, as a con- sequence of nucleotide changes inside or outside of a given cistron. An activated Ds can also cause chro- mosome breakage, which may yield deletions or gen-erate a breakage-fusion-bridge cycle (q.v.). It is now known that Ac is a 4,500 bp segment of DNA that encodes a transposable element (q.v.) which con- tains within it the locus of a functional transposase (q.v.). The transposase gives Ac the ability to detach.