Thymidylate synthase gene expression is stimulated by some (but not all) introns

We previously described the construction of an intronless mouse thymidylate synthase (TS) minigene that has the normal 5' and 3' flanking regions of the gene linked to full length TS cDNA. Transfection of the minigene into ts- hamster V79 cells led to low level expression of normal mouse TS mRNA and protein. In the present study we analyzed the effect of introns on the expression of the TS minigene in transient transfection assays. Inclusion of introns 5 and 6 at their normal locations in the coding region led to an 8-9-fold stimulation of the level of TS and TS mRNA. Almost all of introns 5 and 6 could be deleted without diminishing the stimulatory effect. Inclusion of intron 3 also stimulated the expression of the minigene, although to a lesser extent than introns 5 and 6. However, inclusion of intron 4 had no stimulatory effect. Analysis of minigenes that contained various combinations of introns revealed that the stimulatory effects of the introns were not additive.

Distinct segments within the enhancer region collaborate to specify the type of leukemia induced by nondefective Friend and Moloney viruses

The nondefective Moloney and Friend murine leukemia viruses induce T-cell lymphomas and erythroleukemias, respectively, after being injected into newborn NFS mice. In previous studies, we showed that the distinct disease specificities of the two viruses could be switched by exchanging a small segment, about 200 nucleotides in length, encompassing their enhancer regions. This segment included the direct repeat sequence and an adjacent GC-rich region of about 20 nucleotides defined in studies of Moloney murine sarcoma virus enhancer-promoter function (L. A. Laimins, P. Gruss, R. Pozzatti, and G. Khoury, J. Virol. 49:183-189, 1984). The direct repeats of Friend and Moloney viruses are identical in a central core sequence of 32 nucleotides but have sequence differences on either side of this core as well as in their GC-rich segments. To determine whether disease specificity resides in part or in all of the direct repeat and GC-rich region, we constructed recombinants between Friend and Moloney viruses within this segment and tested them for their disease-inducing phenotypes. We found that disease specificity, in particular the ability of Friend virus sequence to confer erythroleukemogenicity on Moloney virus, is encoded throughout the region in at least three separable segments: the 5' and 3' halves of the direct repeat and the GC-rich segment. When just one of these segments (either both 5' halves of the direct repeat, both 3' halves, or just the GC-rich segment) from Friend virus was substituted into a Moloney virus genome, it conferred only a negligible or low incidence of erythroleukemia (less than or equal to 5% to between 10 and 15%). Any two segments together were considerably more potent (35 to 95% erythroleukemia), with the most effective pair being the two halves of the direct repeat. Individual segments and pairs of segments were considerably more potent determinants when they were matched with a genome of the same origin. Thus, although sequences outside the enhancer region are minor determinants of disease specificity when the enhancer is derived entirely from either Friend or Moloney virus, they can play a significant role when the enhancer is of mixed origin. Some recombinant enhancers conferred a long latent period of disease induction. This was particularly striking when the 5' halves of each copy of the direct repeat sequence were derived from Moloney virus and the 3' halves were derived from Friend virus.(ABSTRACT TRUNCATED AT 400 WORDS)

Rhythms of serum melatonin in patients with spinal lesions at the cervical, thoracic or lumbar region

The neural pathway essential for the diurnal rhythm of serum melatonin was studied in humans. Blood samples from 17 patients with chronic lesions of cervical (n = 8), low thoracic or lumbar (n = 9) spinal cord were collected at 0200, 0400, 1000 and 1400 h of their normal light-dark cycle. Blood samples were also collected from eight control subjects at 0200 and 1400 h. No special treatment of food, drug or photoperiod was implemented. Serum melatonin was extracted by dichloromethane and determined by radioimmunoassay. In patients with cervical spinal lesions (C3-C7), it was found that serum melatonin levels were low and no diurnal rhythm was observed. Conversely, diurnal rhythm of circulating melatonin with significantly higher levels (P less than 0.01) in the dark period were observed in individuals with injuries at the low thoracic or lumbar regions (T9-L2). In the second experiment, blood samples from two other patients were collected for three consecutive days during acute period of spinal injuries (cervical or upper thoracic region) and serum melatonin concentrations were determined. Again, there were low levels of serum melatonin with no observable diurnal rhythm in the patient with cervical lesion (C4-5). However, diurnal rhythms were maintained with high levels in the dark period in the patient with upper thoracic spinal (T2-3) transection. Our data suggest that the cervical region of the spinal cord is part of the neural pathway essential for the diurnal rhythm of pineal melatonin secretion in human beings.

Frequency specificity in intercellular communication. Influence of patterns of periodic signaling on target cell responsiveness

Cells often communicate by means of periodic signals, as exemplified by a large number of hormones and by the aggregation of Dictyostelium discoideum amebas in response to periodic pulses of cyclic AMP. Periodic signaling allows bypassing the phenomenon of desensitization brought about by constant stimuli. To gain further insight into the efficiency of pulsatile signaling, we analyze the effect of periodic stimulation on the dynamic behavior of a receptor system capable of desensitization toward its ligand. We first show that the receptor system adapts to square-wave stimuli, i.e., the response eventually reaches a steady, periodic pattern after a transient phase. By analyzing the dependence of the response on the characteristics of the square-wave stimulation, we show that there exist a waveform and a period of that signal that result in maximum responsiveness of the target system. Similar results are obtained when the signal takes the more realistic form of a periodically repeated stimulation followed by exponential decay of the ligand. The results are discussed with respect to the role of pulsatile secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus and of periodic signaling by cyclic AMP pulses in Dictyostelium. The analysis accounts for the existence, in both cases, of an optimal frequency and waveform of the periodic stimulus that correspond to maximum target cell responsiveness.

Indirect effect of casein phosphopeptides on calcium absorption in rat ileum in vitro

Casein phosphopeptides (CPP) are the phosphorylated fragments of bovine milk casein. They are believed to enhance intestinal absorption of calcium by their ability to form soluble complexes with calcium thereby inhibiting the precipitation of phosphate-calcium salts. In order to evaluate whether they also act in an additional direct manner on the intestinal mucosa, these peptides were added in a phosphate-free medium at a concentration of 1, 2, or 4 mg/ml on the mucosal side of rat ileum mounted in an Ussing chamber in vitro. No effect on the electrical parameters of the tissue was observed. The unidirectional mucosal-to-serosal flux of calcium was significantly reduced in the presence of the peptides, without alteration in the serosal-to-mucosal flux. Jms was 51.71 +/- 2.67 microEq/h.cm2 for control vs. 19.23 +/- 3.95 in the presence of 4 mg/ml CPP. This effect was associated with a reduction in free calcium in the mucosal reservoir of the Ussing chamber, without modification of mucosal total calcium or of serosal total and free calcium. These results indicate that CPP did not directly act on rat ileum to enhance calcium absorption. These peptides bind calcium, and the CPP--calcium complex which was not efficiently absorbed remained on the mucosal side of the tissue. In these conditions, the physiological role of CPP on intestinal calcium absorption could be only an indirect luminal inhibition of the precipitation of phosphate-calcium salts. This effect remains to be clearly established.

[Trace iron and free radical analysis in intraocular ferreous foreign body injury]

Within 7 days after implantation of ferreous foreign body into the vitreous of rabbit eyes, ERG amplitudes decreased and local retinal degeneration was discernible under light and electron microscope. The iron ion concentration in aqueous and vitreous of the experimental eyes began to increase 4 hours after implantation and became significantly higher than that in the control eyes after 24 hours (P less than 0.01 or 0.05). The value of malondialdehyde (MDA) in the retina was closely correlated with the iron concentration in the vitreous (r = 0.86). 24 hours after iron implantation, the level of MDA in the experimental eyes was 1.3 times that in the control eyes (P less than 0.05). On the 28th day, the level of MDA was about 2 times that in the control eyes. The clinical significance of early trace iron analysis of the aqueous and the mechanism of retinal degeneration were discussed.

Embryonic entorhinal transplants project selectively to the deafferented entorhinal zone of adult mouse hippocampi, as demonstrated by the use of Thy-1 allelic immunohistochemistry. Effect of timing of transplantation in relation to deafferentation

The mouse Thy-1.1/Thy-1.2 allelic marking system is used to show that transplanted embryonic entorhinal cortex can reinnervate adult host hippocampi. The projection is limited to the appropriate terminal zones--viz. the outer two-thirds of the stratum moleculare of the dentate gyrus, and the stratum lacunosum-moleculare of the hippocampus--and extends for up to about 2 mm into the denervated host terminal field. The reconstruction of the entorhinal projections to the host requires direct contact between the embryonic donor tissue and the denervated adult host terminal field, and is dependent upon removal of the ipsilateral host entorhinal area. In the absence of an overall deafferenting host entorhinal lesion the transplanted entorhinal area forms only small local projections which are confined to areas which would have been locally deafferented as a result of direct damage to the host entorhinal afferents (i.e. during their intrahippocampal course) by the hippocampal lesion caused at the time when the transplant was inserted. The correct relative timing of deafferentation and transplantation is vital for the formation of the transplant-to-host projection. The host dendrites can be made receptive to entorhinal transplant projections by removal of the host entorhinal area at the time of transplantation. When deafferentation is performed first and transplantation is delayed, it is found that the deafferented host dendrites retain this receptivity even when deafferentation has been performed as much as two months before transplantation. Reversing the order of transplantation and deafferentation, however, shows that the transplants have only a transient ability to project to the deafferented host territory. Thus, transplants inserted and allowed to become established for one week before host deafferentation make very much reduced projections to the host, and from two weeks onwards are incapable of any detectable response to subsequent removal of the host entorhinal area. Coextensive with the formation of transplant-to-host entorhinodentate projections, the host entorhinal lesion also induces an intensification of the acetylcholinesterase staining of the host septodentate afferents in the denervated outer dentate stratum moleculare. The findings demonstrate the accurate reconstruction of a lost projection in adult brain by transplanting the appropriate type of embryonic tissue, but the results of altering the relative timing of deafferentation and transplantation raise currently unsolved questions about the nature of the competitive interactions between transplant and host axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Lophotoxin and related coral toxins covalently label the alpha-subunit of the nicotinic acetylcholine receptor

Lophotoxin and lophotoxin analog-1 are uncharged cyclic diterpenes obtained from gorgonian corals. They have been shown to block the function of nicotinic acetylcholine receptors. Inhibition results from blockade of the agonist recognition site and appears irreversible in that extensive washing does not restore receptor function. This study was undertaken to determine whether this apparently irreversible inhibition involves covalent labeling at a selective site and to further characterize this site directly. Incubation of membranes prepared from the electric organ of Torpedo californica with analog-1 followed by reduction with NaB3H4 resulted in the incorporation of radioactivity into several membrane proteins. The incorporation of radioactivity into the alpha-subunit of the receptor was blocked by prior incubation with agonists and antagonists. [3H]Lophotoxin and [3H]analog-1 were prepared by reduction with NaB3H4 and back-oxidation with CrO3. The radiolabeled coral toxins reacted selectively and covalently with the alpha-subunit of the receptor. Their binding was prevented by prior exposure to agonists and antagonists. In contrast to the site-directed alkylating agent 4-(N-maleimido)benzyltrimethylammonium iodide, prior reduction of the receptor was not required for covalent binding of 3H-labeled coral toxins. Selective reduction of Cys192 and Cys193 followed by alkylation with 4-(N-maleimido)benzyltrimethylammonium iodide blocked the binding of [3H]analog-1, whereas alkylation with iodoacetic acid or iodoacetamide did not. Thus, the binding site for the coral toxins does not overlap the binding surface near Cys192 and Cys193. Digestion of isolated labeled alpha-subunits with endoglycosidase H revealed that the polypeptide portion of the protein retained the covalently bound [3H]analog-1. Digestion with staphylococcal V8 protease revealed two major peptides of approximately 19 and 20 kDa, along with several smaller peptides. Only the 20-kDa peptide retained the covalently bound [3H]analog-1, localizing the site of covalent attachment between Ser173 and Glu335. The unique chemical structure and covalent reactivity of these gorgonian coral toxins will undoubtedly allow further insights into the structure of the agonist recognition site.

Characterization of infectious molecular clones of simian immunodeficiency virus (SIVmac) and human immunodeficiency virus type 2: persistent infection of rhesus monkeys with molecularly cloned SIVmac

Infection of macaque monkeys with simian immunodeficiency virus (SIV) is probably the best animal model currently available for studying acquired immunodeficiency syndrome. In this report, we describe three infectious molecular clones of SIVmac and one of human immunodeficiency virus type 2 (HIV-2) and their use in the study of cell and species specificity, animal infection, and the relationship of gene sequence to function. Replication of the cloned viruses in different cell lines varied dramatically. Some human CD4+ cell lines (HUT 78 and MT-4) supported the replication of SIVmac and HIV-2, while others (CEM and Jurkat-T) supported the replication of HIV-2 but not SIVmac. Growth of cloned virus in macaque lymphocytes in vitro was predictive of macaque infection in vivo. Macaque lymphocytes supported the replication of SIVmac239 and SIVmac251 but not SIVmac142 or HIV-2ROD. Using virus recovery and antibody response as criteria for infection, macaques that received cloned SIVmac251 and SIVmac239 became infected, while macaques receiving cloned SIVmac142 and HIV-2ROD did not become infected. Nucleotide sequences from the envelope region of all four cloned viruses demonstrated that there is considerable flexibility in the location of the translational termination (stop) signal. These infectious molecular clones will be very useful for future studies directed at the molecular basis for persistence, pathogenicity, tropism, and cell and species specificity.

Optical implementation of binary symmetric logic functions

Optical implementations of binary multiple-variable symmetric logic functions are proposed. By using a triangular array of lossless beam splitters together with optical on-off switches, an optical binary symmetric logic module (OBSLM) is experimentally implemented. The applications of the OBSLM to optical digital, symbolic, and neural computing are discussed.

Inversely related evolution of growth hormone and prolactin secretions in long-term tissue cultures of human pituitary adenomas from acromegalic patients

Pituitary tumoral tissue from 20 acromegalic patients was cultured for up to 120 d in a medium containing 5 nM cortisol. In all cultures, growth hormone (GH) release decreased. At the beginning of the culture, prolactin (PRL) was detected in 18 adenomas, varying from 0.5 to 1000 ng per flask per day. Thereafter, in 10 cases PRL secretion increased from 3 to 50 times the basal level, most frequently after a lapse of 9 to 30 d. PRL secretion remained low in three cases, undetectable in one case only. When added at 350 nM, cortisol increased GH secretion up to 20-fold and simultaneously decreased PRL secretion by as much as 10% of the basal level. Withdrawing cortisol reversed the situation. Immunocytochemical studies of the tumor at surgery showed, besides GH immunoreactive (IR) cells, PRL-IR cells (from rare cells to 10% of total cells) in 15 adenomas, correlating with the first days of culture PRL levels. In cultured explants, mitoses were never found. In 5 nM cortisol medium, the number of GH-IR cells decreased and PRL-IR cells increased or appeared. With 350 nM cortisol, the number of GH-IR cells increased, and PRL-IR cells were scarce or absent. Immunoreactivities for GH and PRL were found in different cells. Care was taken to exclude cultures containing normal pituitary tissue, and because no mitoses were found, these results suggest that most somatotropic adenomas can reversibly shift their secretion from GH to PRL in culture. This capacity to secrete PRL, hidden or low in vivo, is revealed by the favorable low cortisol conditions present in vitro.

Simian immunodeficiency virus from African green monkeys

Simian immunodeficiency virus (SIV) was isolated from the total peripheral blood mononuclear cell population and the monocyte-macrophage adherent cell population of three seropositive green monkeys originating from Kenya. SIV from these African green monkeys (SIVagm) was isolated and continuously produced with the MOLT-4 clone 8 (M4C18) cell line but not with a variety of other cells including HUT-78, H9, CEM, MT-4, U937, and uncloned MOLT-4 cells. Once isolated, these SIVagm isolates were found to replicate efficiently in M4C18, SupT1, MT-4, U937, and Jurkat-T cells but much less efficiently if at all in HUT-78, H9, CEM, and MOLT-4 cells. The range of CD4+ cells fully permissive for replication of these SIVagm isolates thus differs markedly from that of previous SIV isolates from macaques (SIVmac). These SIVagm isolates had a morphogenesis and morphology like that of human immunodeficiency virus (HIV) and other SIV isolates. Antigens of SIVagm and SIVmac cross-reacted by comparative enzyme-linked immunosorbent assay only with reduced efficiency, and optimal results were obtained when homologous antibody and antigen were used. Western blotting (immunoblotting) of purified preparations of SIVagm isolate 385 (SIVagm385) revealed major viral proteins of 120, 27, and 16 kilodaltons (kDa). The presumed major core protein of 27 kDa cross-reacted antigenically with the corresponding proteins of SIVmac (28 kDa) and HIV-1 (24 kDa) by Western blotting. Hirt supernatant replicative-intermediate DNA prepared from cells freshly infected with SIVagm hybridized to SIVmac and HIV-2 DNA probes. Detection of cross-hybridizing DNA sequences, however, required very low stringency, and the restriction endonuclease fragmentation patterns of SIVagm were not similar to those of SIVmac and HIV-2. The nucleotide sequence of a portion of the pol gene of SIVagm385 revealed amino acid identities of 65% with SIVmac142, 64% with HIV-2ROD, and 56% with HIV-1BRU; SIVagm385 is thus related to but distinct from previously described primate lentiviruses SIVmac, HIV-1, and HIV-2. Precise information on the genetic makeup of these and other SIV isolates will possibly lead to better understanding of the history and evolution of these viruses and may provide insight into the origin of viruses that cause acquired immunodeficiency syndrome in humans.

Reduced lipid peroxidation in dilated hearts of cardiomyopathic turkeys

Adverse pulmonary reactions to some nitrofuran antibiotics are thought, in part, to involve production of reactive oxygen radicals. Furazolidone, a nitrofuran antibiotic, causes a dilated cardiomyopathy in domestic turkeys. The mechanism of this drug induced cardiomyopathy is unknown. We investigated the possible role of free radical injury in this heart failure model. Left ventricular lipid peroxidation capacity, assessed by two methods (the thiobarbituric acid reactive substances and lipid hydroperoxides assays respectively), was investigated in five 5-8 week old cardiomyopathic turkeys with severe cardiac dilatation, left ventricular dysfunction and systemic hypotension, and in five control birds. Superoxide dismutase activity, total and manganese, was also measured in the crude left ventricular homogenates. Both lipid peroxidation products were reduced in the myopathic hearts: thiobarbituric acid reactive substances (malondialdehyde) 70(SEM 4) v 86(3) nmol.100 mg protein-1 in controls, p less than 0.02; and lipid hydroperoxides 29(7) v 74(14) nmol.100 mg protein-1, p less than 0.02. Total superoxide dismutase activity was similar in cardiomyopathic and control hearts: 670(26) v 657(105) nitrite units.100 mg protein-1. Although total superoxide dismutase activity was unchanged, we found decreased manganese superoxide dismutase in the dilated hearts compared with controls (54% v 84% of total activity, p less than 0.02). In separate in vitro experiments furazolidone (2-10 mg.g wet weight-1) did not increase malondialdehyde production in turkey (or rat) left ventricular homogenates. These results indicate that cardiomyopathy induced by furazolidone is associated with decreased myocardial lipid peroxidation. Although as yet unexplained, the decrease may be due to a diminished amount of heart lipid susceptible to peroxidation accompanying the process of cardiac hypertrophy and dilatation.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple hepatic trans-acting factors are required for in vitro transcription of the human alpha-1-antitrypsin gene

The human alpha-1-antitrypsin (AAT) gene is expressed in the liver, and its deficiency causes pulmonary emphysema. We have demonstrated that its 5'-flanking region contains cis-acting elements capable of directing proper transcription in the presence of rat liver nuclear extract. The in vitro transcription system is tissue-specific in that the AAT promoter is functional in nuclear extracts prepared from the liver but not from HeLa cells. Experiments in which rat liver and HeLa nuclear extracts were mixed suggested the presence of a specific activator(s) in hepatocytes rather than a repressor(s) in nonproducing cells. Two protected regions were detected in the promoter by DNase I footprinting analysis with rat liver nuclear extracts. Region one spanned -78 to -52 and region two spanned -125 to -100 in the 5'-flanking sequence of the gene. By gel retardation assays with synthetic oligonucleotides, at least two distinct liver nuclear factors were identified, HNF-1 and HNF-2 (hepatocyte nuclear factors), which bound specifically to the first and second region, respectively. We present evidence that HNF-1 and HNF-2 are positively acting, tissue-specific transcription factors that regulate hepatic expression of the human AAT gene.

Free-space folded-path optical programmable logic array

A new method to realize a medium-scale, free-space optical programmable logic array is proposed. By using either a two-dimensional optical spatial light modulator or an array of one-dimensional spatial light modulators inside a lens-based multiple-beam-path cavity, an array of optical multiple-variable logic product terms is generated. This device, together with a programmable multiple-variable OR matrix, can be used to implement any Boolean combinatorial logic operations. For an optical binary combinatorial logic computation, the proposed method efficiently uses three-dimensional space and optical elements. Preliminary experimental results obtained using an inexpensive liquid-crystal television are included.

Topological localization of a segment of the eel voltage-dependent sodium channel primary sequence (AA 927-938) that discriminates between models of tertiary structure

Antibodies were raised against a synthetic peptide corresponding to residues 927-938 of the eel electroplax sodium channel primary sequence. This segment, lying between putative internal repeat domains II and III, is postulated to be exposed on the cytoplasmic surface of the membrane in several recent models of channel tertiary structure and on the external surface in another. The antiserum and affinity-purified IgG derived from it specifically recognize the peptide and the eel sodium channel in a solid-phase radioimmunoassay and bind to a single diffuse band of 260-280 kDa on Western blots of eel electroplax membrane proteins. All reactions are blocked by co-incubation of the antibodies with the synthetic peptide (1 microM). The antibody immunoprecipitates the solubilized channel in a form that retains its characteristic high-affinity binding of saxitoxin. In eel electroplax, the antibodies label only the innervated membrane known to contain sodium channels; at the ultrastructural level, this labeling is exclusively associated with the cytoplasmic surface of the membrane. Sodium channels containing the epitope are not seen in the postsynaptic membrane or in the membrane of the presynaptic nerve terminal. Segment 927-938 of the eel sodium channel is accessible on the surface of the protein in its solubilized form and is exposed in the cytoplasmic face of the innervated membrane of the electroplax in situ. This location is consistent with 3 models of channel structure but not with a fourth.

Site-specific mutations in vectors that express antigenic and temperature-sensitive phenotypes of the M gene of vesicular stomatitis virus

Full-length cDNA copies of mRNAs coding for the matrix (M) proteins of vesicular stomatitis virus and its mutant tsO23(III) were cloned in pBSM13- (BlueScribe). The authenticity of these clones was demonstrated by restriction enzyme mapping, DNA sequencing, and in vitro transcription and translation to identify the two M proteins by Western immunoblotting with epitope-specific monoclonal antibodies. Site-directed mutants were constructed by primer extension of synthetic oligodeoxynucleotides with one or two nucleotide changes to alter the glycine at amino acid 21 of the wild-type (wt) M gene to glutamic acid, alanine, or proline. Similarly, a revertant was created in the M gene of mutant tsO23 by a Glu-21----Gly substitution. A series of wt- and mutant-M-gene chimeras was also constructed to create mutant and revertant clones with Leu----Phe and His----Tyr alterations at amino acids 111 and 227, respectively. We then moved the wt and tsO23 M genes and their site-specific mutants and chimeras cloned in pBSM13- into the eucaryotic expression vector pTF7 directed by the T7 bacteriophage RNA polymerase of the vaccinia virus recombinant vTF1-6,2. Western blot analysis of the M proteins transiently expressed in CV-1 cells by plasmids carrying M genes altered at amino acid 21 revealed that the critical antigenic determinant (epitope 1) is expressed only by the Gly-21 M protein and not by Glu-21, Ala-21, or Pro-21 M proteins. Of particular interest is an apparent conformational change, evidenced by slightly but significantly retarded electrophoretic migration, in plasmid-expressed M proteins with amino acids substituted for glycine at position 21. The glutamic acid at position 21 of tsO23 is not responsible for its temperature-sensitive phenotype, because a tsO23 revertant plasmid with glycine substituted at position 21 fails to rescue tsO23 virus in cells infected at the restrictive temperature; conversely, plasmids expressing wt M protein with substitutions of glutamic acid, alanine, or proline at position 21 are just as effective in marker rescue of tsO23 as is the Gly-21 wt M protein. Marker rescue experiments with wt- and mutant-M-gene chimeras support the hypothesis of K. Morita, R. Vanderoef, and J. Lenard (J. Virol. 61:256-263, 1987) that the temperature-sensitive phenotype of tsO23 is due to a phenylalanine substituted for leucine at amino acid 111, rather than the His-227----Tyr substitution or the Gly-21----Glu substitution, which independently accounts for the loss of epitope 1 in the mutant M protein of tsO23.(ABSTRACT TRUNCATED AT 400 WORDS)