No abstract

A method was devised for identifying nonlethal mutants of T4 bacteriophage which lack the capacity to induce degradation of the deoxyribonucleic acid (DNA) of their host, Escherichia coli. If a culture is infected in a medium containing hydroxyurea (HU), a compound that blocks de novo deoxyribonucleotide biosynthesis by interacting with ribonucleotide reductase, mutant phage that cannot establish the alternate pathway of deoxyribonucleotide production from bacterial DNA will fail to produce progeny. The progeny of 100 phages that survived heavy mutagenesis with hydroxylamine were tested for their ability to multiply in the presence of HU. Four of the cultures lacked this capacity. Cells infected with one of these mutants, designated T4nd28, accumulated double-stranded fragments of host DNA with a molecular weight of approximately 2 x 108 daltons. This mutant failed to induce T4 endonuclease II, an enzyme known to produce single-strand breaks in doublestranded cytosine-containing DNA. The properties of nd28 give strong support to an earlier suggestion that T4 endonuclease II participates in host DNA degradation. The nd28 mutation mapped between T4 genes 32 and 63 and was very close to the latter gene. It is, thus, in the region of the T4 map that is occupied by genes for a number of other enzymes, including deoxycytidylate deaminase, thymidylate synthetase, dihydrofolate reductase, and ribonucleotide reductase, that are nonessential to phage production in rich media.

In the mammalian central nervous system (CNS) there are literally billions of neuronal synapses with each employing a particular neurotransmitter. A neurotransmitter can be either excitatory or inhibitory, depending on whether it produces depolarization or hyperpolarization of the neuronal membrane, respectively. Furthermore, the duration of transmitter action either may be in the low millisecond time range or it may show a prolonged latency and duration of action spanning tenths of seconds or seconds. An attractive model for neuronal organization suggests that there is considerable specificity of the various transmitters for these differing roles.1 While such "classical neurotransmitters" as acetylcholine and the catecholamines, dopamine and norepinephrine, may show either excitatory or inhibitory actions, they also, in most if not all cases, act in the CNS by mechanisms that are of prolonged duration and serve to modulate the intensity of millisecond excitatory or inhibitory impulses.2 Escalating evidence suggests that the neurotransmitter receptors for these systems are coupled to gated ion channels via guanine nucleotide-binding proteins (G proteins) and second messenger intermediates. This mechanism provides great flexibility for adjusting the intensities and durations of many different stimuli. In contrast, millisecond neurotransmitters act on receptors that are coupled to a gated ion channel located on the same transmembrane protein molecule. This arrangement allows microsecond triggering of channel opening in response to binding of the effector molecule. Like the slow-acting modulatory neurotransmitters, millisecond neurotransmitters may also exhibit inhibitory or excitatory effects. For example, the neurotransmitters -aminobutyric acid (GABA) and glycine are thought to be millisecond inhibitory neurotransmitters. However, the most prevalent neurotransmitter class in the mammalian CNS would appear to be the millisecond excitatory neurotransmitters. Foremost among the candidates for millisecond excitatory neurotransmitters in the CNS are the acidic amino acids Lglutamic acid (1) and L-aspartic acid (2).3-6 Glutamic acid, in particular, has been shown to satisfy many of the criteria required for a substance to be designated as a neurotransmitter.7•8 It has been over 30 years since Hayashi9 first reported the convulsant effects of L-glutamic acid and over 25 years