Lipid-Induced Modulation of the Protein Packing in Two-Dimensional Crystals of Bacteriorhodopsin

The mechanism by which bacteriorhodopsin (BR), the light-driven proton pump from the purple membrane (PM) of Halobacterium halobium, arranges in a 2D hexagonal array has been studied by reconstitution of BR in complexes of two types of bilayer made either with PM-derived lipids or with PM lipids and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The unit cell dimensions of the 2D protein crystals, determined by correlation averaging analysis of freeze-fracture electron micrographs, were compared with the lattice constant of the PM. In complexes made with delipidated BR and with the polar lipids extracted from H. halobium cells (HHPL), BR trimers are arranged in a hexagonal lattice with the same lattice constant of 5.9 ± 0.2 nm as found in the PM. In BR-containing complexes made with PM-derived lipids and DMPC at several protein:lipid mole ratios, BR trimers are also arranged in a hexagonal lattice, but with a unit cell dimension of 9.2 ± 0.2 nm, which is about one-third larger compared to that measured in PM (Michel et al. , 1980). In a subclass of this type of complexes, orthogonal BR arrays were observed with a lattice constant of 5.9 × 9.9 ± 0.2 nm. It appears that insertion of DMPC into the BP/PM-derived lipid complexes increases the center-to-center distances in both array types by a discrete amount.     

An Intriguing Controversy over Protein Structural Class Prediction

A recent report by Bahar et al. [(1997), Proteins 29, 172–185] indicates that the coupling effects among different amino acid components as originally formulated by K. C. Chou [(1995), Proteins 21, 319–344] are important for improving the prediction of protein structural classes. These authors have further proposed a compact lattice model to illuminate the physical insight contained in the component-coupled algorithm. However, a completely opposite result was concluded by Eisenhaber et al. [(1996), Proteins 25, 169–179], using a different dataset constructed according to their definition. To address such an intriguing controversy, tests were conducted by various approaches for the datasets from an objective database, the SCOP database [Murzin et al. (1995), J. Mol. Biol. 247, 536–540]. The results obtained by both self-consistency and jackknife tests indicate that the overall rates of correct prediction by the algorithm incorporating the coupling effect among different amino acid components are significantly higher than those by the algorithms without counting such an effect. This is fully consistent with the physical reality that the folding of a protein is the result of a collective interaction among its constituent amino acid residues, and hence the coupling effects of different amino acid components must be incorporated in order to improve the prediction quality. It was found by a revisiting the calculation procedures by Eisenhaber et al. that there was a conceptual mistake in constructing the structural class datasets and a systematic mistake in applying the component-coupled algorithm. These findings are informative for understanding and utilizing the component-coupled algorithm to study the structural classes of proteins.     

Identification of a component of rat mononuclear cell SRS as leukotriene D

Slow reacting substance (SRS), produced by rat peritoneal mononuclear cells after stimulation with ionophore A23187, consists of two main components (Bach, M.K. et al. (1979) J. Immunol. 122, 160–165). One of these components was recently identified as leukotriene C-1. The other component has now been identified as leukotriene D.     

Neurotensin receptors: Binding properties,transduction pathways,and structure

Summary Neurotensin is a 13-amino acid peptide (pGlu-Leu-Tyr-Glu-Asn-Lys-Pro-Arg-Arg-Pro-Tyr-Ile-Leu) originally isolated from hypothalami (Carraway and Leeman, 1973) and later from intestines (Kitabgiet al., 1976) of bovine. The peptide is present throughout the animal kingdom, suggesting its participation to important processes basic to animal life (Carrawayet al., 1982). Neurotensin and its analogue neuromedin-N (Lys-Ile-Pro-Tyr-Ile-Leu) (Minaminoet al., 1984) are synthesized by a common precursor in mammalian brain (Kislauskiset al., 1988) and intestine (Dobneret al., 1987). The central and peripheral distribution and effects of neurotensin have been extensively studied. In the brain, neurotensin is exclusively found in nerve cells, fibers, and terminals (Uhlet al., 1979), whereas the majority of peripheral neurotensin is found in the endocrine N-cells located in the intestinal mucosa (Orciet al., 1976; Helmstaedteret al., 1977). Central or peripheral injections of neurotensin produce completely different pharmacological effects (Table I) indicating that the peptide does not cross the blood-brain barrier. Many of the effects of centrally administered neurotensin are similar to those of neuroleptics or can be antagonized by simultaneous administration of TRH (Table I). The recently discovered nonpeptide antagonist SR 48692 (Gullyet al., 1993) can inhibit several of the central and peripheral effects of neurotensin (Table I).Like many other neuropeptides, neurotensin is a messenger of intracellular communication working as a neurotransmitter or neuromodulator in the brain (Nemeroffet al., 1982) and as a local hormone in the periphery (Hirsch Fernstromet al., 1980). Thus, several pharmacological, morphological, and neurochemical data suggest that one of the functions of neurotensin in the brain is to regulate dopamine neurotransmission along the nigrostriatal and mesolimbic pathways (Quirion, 1983; Kitabgi, 1989). On the other hand, the likely role of neurotensin as a parahormone in the gastrointestinal tract has been well documented (Rosell and Rökaeus, 1981; Kitabgi, 1982).Both central and peripheral modes of action of neurotensin imply as a first step the recognition of the peptide by a specific receptor located on the plasma membrane of the target cell. Formation of the neurotensin-receptor complex is then translated inside the cell by a change in the activity of an intracellular enzyme. This paper describes the binding and structural properties of neurotensin receptors as well as the signal transduction pathways that are activated by the peptide in various target tissues and cells.     

Origin of replication, oriC, of the Escherichia coli chromosome: mapping of genes relative to R.EcoRI cleavage sites in the oriC region.

Summary A precise genetic-physical map of the tnailv region at 82 min on the genetic map of E. coli is obtained through deletion mapping and analysis by restriction endonuclease EcoRI of plasmids, derived from an F carrying the genes between aroE and ilv.A locus, designated het, which in its diploid state results in slow growth and heterogeneity of cell size due to distorted cell division, maps between bglB and asn, 30–45 kb counterclockwise of ilv.The pattern of R.EcolRI cleavage sites in the het region is identical with the pattern obtained by Marsh and worcel (1977) who analyzed DNA labeled preferentially in the region of the DNA replication origin (oriC). We suggest that oriC is identical with the het site and that it can be allocated to a position 32 kb counterclockwise of the ilv operon.Abbreviations CCC covalently closed circular - kb kilobases - MD 10⁶ Daltons - mw molecular weight - R.EcoRI restriction endonuclease EcoRI New Genetic Symbols het heterogeneity of cell size distribution (this study) - maf maintenance of F (Wada et al., 1977) - oriC chromosomal origin of replication (Hiraga, 1976) - oriV origin of vegetative replication of F (Guyer et al., 1976) - oriT origin of transfer replication of F (Guyer et al., 1976) - poh permissive on Hfr (Hiraga, 1976)     

Myelin basic protein undergoes a broader range of modifications in mammals than in lower vertebrates

Myelin basic protein (MBP) is an important component of the myelin sheath surrounding neurons, and it is directly affected in demyelinating diseases. MBP contains a relatively large number of post-translational modifications (PTMs), which have been reported to play a role in multiple sclerosis, while MBPs from lower vertebrates have been reported to be incapable of inducing multiple sclerosis or allergic encephalitis. This study reveals the extent of differences in PTM patterns for mammalian and nonmammalian MBPs. This included intact mass and de novo sequence analysis of approximately 85% of rattlesnake MBP, the first reptile MBP to be characterized, and of bovine MBP. We identified 12 PTMs at 11 sites in the five bovine MBP charge components, which include both previously reported and novel modifications. The most notable modification is an acetylation of lysine 121. Other modifications found in bovine MBP include N-terminal acetylation in components C1, C2, and C3; oxidation of methionine 19 in all five components; all charge isomers having both a mono- and dimethylated (symmetric) arginine at position 106; deimination in arginines 23 and 47 found only in component C8b; deimination of arginine 96 and deamidation in glutamine 102 found in components C2, C3, C8a, and C8b; phosphorylation in threonine 97 restricted to charge components C2 and C3; deimination in arginine 161 only found in component C3; deamidation of glutamine 120 was only observed in C3. All four deiminated arginines and one acetylated lysine were first experimentally revealed in this study for bovine MBP. Mascot database searching combined with de novo sequence analysis of rattlesnake MBP provided more than 85% sequence coverage. A few PTMs were also revealed in rattlesnake MBP: mono- and dimethylated Arg, protein N-terminal acetylation, and deiminated Arg. Overall, snake MBP was found to undergo less modification than bovine MBP on the basis of the mass heterogeneity of the intact protein, the bottom-up structure analysis, and the limited complexity of rattlesnake MBP chromatography. The combined data from this study and information from previous studies extend the known MBP PTMs, and PTMs unique to higher vertebrates are proposed.     

A model for avian primary moult

The regression methods frequently used to estimate the parameters associated with primary moult in birds are unsatisfactory. Results obtained using least squares regression, and various ad hoc adaptations, are so obviously incorrect that many authors have fitted lines ‘by eye’ (Newton 1968, Thomas & Dartnall 1971, Elliott et al. 1976, Morrison 1976, Appleton & Minton 1978). In a comparison of seven regression methods, estimates of the average starting date varied between 29 June and 31 July, completion date between 2 and 24 October, and duration of moult between 72 and 109 days for the Redshank Tringa totanus, in spite of the very large sample of 1482 observations (Summers et al. 1983). In this paper we present a new approach to the analysis of primary moult and develop a mathematical model specifically designed for moult data.     

Chromosome differences in Peromyscus maniculatus populations at different altitudes in Colorado

Mice of the genus Peromyscus all have 48 chromosomes. Yet the appearance of the 48 chromosomes is highly variable from species to species (Hsu & Arrighi, 1966, 1968, 1971; Pathak et al., 1973) and even in different populations of the same species (Sparkes & Arakaki, 1966; Ohno et al., 1966; Hsu & Arrighi, 1968; Arakaki et al. 1970; Te & Dawson, 1971; Bradshaw & Hsu, 1972; Murray & Kitchin, 1976). The evolutionary significance of this variation and the mechanisms for its initiation and maintenance have been of interest for quite a few years. However, it was not until the sophisticated chromosome banding techniques became available that mammalian cytogeneticists were able to begin to study the chromosome variation of Peromyscus in some detail. The use of C-banding led Hsu & Arrighi (1971) to the finding that the short arms of chromosomes in three different species of Peromyscus contained constitutive heterochromatin. These results suggested that the variations in the number of acrocentric chromosomes in Peromyscus might be a result of different amounts of heterochromatin. Later studies (Duffey, 1972; Waterbury, 1972; and Pathak et al., 1973) were also consistent with this hypothesis.However, it was soon discovered that not all chromosomal differences among Peromyscus populations are due to heterochromatin changes. Studies by Arighi et al. (1976) and Murray & Kitchin (1976) showed that some chromosomal differences between species and subspecies of Peromyscus are due to pericentric inversions. Thus, it appears that both inversions and the addition of heterochromatin are involved in the evolution of the karyotype of Peromyscus.The purpose of our study was to investigate the chromosomes of Peromyscus maniculatus in different populations in Colorado (U.S.A.) and to test for relationships involving an altitudinal gradient. In the first part of this study, orcein stained chromosomes from three subspecies of mice sampled at nine different altitudes were examined for karyotype variability. In the second part of the study, karyotypes of two subspecies (P. m. rufinus and P. m. luteus), representing high and low altitude populations were examined with Q banding to determine the mechanisms responsible for chromosomal differences.     

Taxonomy and geographical distribution of Dugesia japonica and D. ryukyuensis in the Far East

Dugesia japonica Ichikawa et Kawakatsu, 1964, is a common and polymorphic species of freshwater planarian distributed widely in the Far East. In 1976 the geographic populations were separated into 2 subspecies (D.j.japonica and D.j. ryukyuensis). The taxonomy of this species is reconsidered once again from the morphological, anatomical, histological, and karyological viewpoints. From the result of these studies, D.j. ryukyuensis is elevated to the rank of species: D. ryukyuensis Kawakatsu, 1976. D. japonica (n = 8, 2x = 16, 3x = 24) differs from D. ryukyuensis (n = 7, 2x = 14, 3x = 21) in having an asymmetrical penis papilla without a well-developed valve surrounding its basal part, and a well-developed vagina (distribution: the Japanese Islands, Taiwan, the Korean Peninsula, China, and Primorskiy, Northeast Siberia, in Russia). D. ryukyuensis is characterized by an asymmetrical penis papilla with a well-developed valve surrounding its basal part, and a less-developed vagina (distribution: the Southwest Islands of Japan).     

Comparison of ion current noise predicted from different models of the sodium channel gating mechanism in nerve membrane

Summary The theoretical power density spectrumS(f) of ion current noise is calculated from several models of the sodium channel gating mechanism in nerve membrane. Sodium ion noise experimental data from the frog node of Ranvier [Conti, F.,et al. (1976),J. Physiol. (London) 262:699] is used as a test of the theoretical results. The motivation for recent modeling has been evidence for a coupling between sodium activation and inactivation from voltage clamp data. The two processes are independent of one another in the Hodgkin and Huxley (HH) model [Hodgkin, A.L., Huxley, A.F. (1952),J. Physiol. (London) 117:500] The noise data is consistent with HH, as noted by Contiet al. (1976). The theoretical results given here appear to indicate that only one case of coupling models is also consistent with the noise data.     

Interactive effects of light,leaf temperature,CO2 and O2 on photosynthesis in soybean

A biochemical model of C ₃photosynthesis has been developed by G.D. Farquhar et al. (1980, Planta 149, 78–90) based on Michaelis-Menten kinetics of ribulose-1,5-bisphosphate (RuBP) carboxylase-oxygenase, with a potential RuBP limitation imposed via the Calvin cycle and rates of electron transport. The model presented here is slightly modified so that parameters may be estimated from whole-leaf gas-exchange measurements. Carbon-dioxide response curves of net photosynthesis obtained using soybean plants (Glycine max (L.) Merr.) at four partial pressures of oxygen and five leaf temperatures are presented, and a method for estimating the kinetic parameters of RuBP carboxylase-oxygenase, as manifested in vivo, is discussed. The kinetic parameters so obtained compare well with kinetic parameters obtained in vitro, and the model fits to the measured data give r ²values ranging from 0.87 to 0.98. In addition, equations developed by J.D. Tenhunen et al. (1976, Oecologia 26, 89–100, 101–109) to describe the light and temperature responses of measured CO₂-saturated photosynthetic rates are applied to data collected on soybean. Combining these equations with those describing the kinetics of RuBP carboxylase-oxygenase allows one to model successfully the interactive effects of incident irradiance, leaf temperature, CO₂ and O₂ on whole-leaf photosynthesis. This analytical model may become a useful tool for plant ecologists interested in comparing photosynthetic responses of different C₃ plants or of a single species grown in contrasting environments.Abbreviations PCO photorespiratory carbon oxidation - PCR photosynthetic carbon reduction - PPFD photosynthetic photon-flux density - RuBP ribulose bisphosphate     

Genetic localization of diuron- and mucidin-resistant mutants relative to a group of loci of the mitochondrial DNA controlling coenzyme QH2-cytochrome c reductase in Saccharomyces cerevisiae.

Summary Diuron-resistance, DIU (Colson et al., 1977), antimycin-resistance, ANA (Michaelis, 1976; Burger et al., 1976), funiculosin-resistance, FUN (Pratje and Michaelis, 1977; Burger et al., 1977) and mucidin-resistance, MUC (Subik et al., 1977) are each coded by a pair of genetic loci on the mit DNA of S. cerevisiae. In the present paper, these respiratory-competent, drug-resistant loci are localized relative to respiratory-deficient BOX mutants deficient in coenzyme QH₂-cytochrome c reductase (Kotylak and Slonimski, 1976, 1977) using deletion and recombination mapping. Three drug-resistant loci possessing distinct mutated allelic forms are distinguished. DIU1 is allelic or closely linked to ANA2, FUN1 and BOX1; DIU2 is allelic or closely linked to ANA1, MUC1 and BOX4/5; MUC2 is allelic to BOX6. The high recombinant frequencies observed between the three loci (13% on the average for 33 various combinations analyzed) suggest the existence of either three genes coding for three distinct polypeptides or of a single gene coding for a single polypeptide but subdivided into three easily separable segments. The resistance of the respiratory-chain observed in vitro in the drug-resistant mutants and the allelism relationships between respiratory-competent, drug-resistant loci and coQH₂-cyt c reductase deficient, BOX, loci strongly suggest that each of the three drug-resistant loci codes for a structural gene-product which is essential for the normal coQH₂-cyt c reductase activity and is obviously a good candidate for a gene product of the drug-resistant loci mapped in this paper. Polypeptide length modifications of cytochrome b were observed in mutants deficient in the coQH₂-cyt c red and localized at the BOX1, BOX4 and BOX6 genetic loci (Claisse et al., 1977, 1978) which are precisely the loci allelic to drug resistant mutants as shown in the present work. Taken together these two sets of data provide a strong evidence in favor of the idea that there exist three non contiguous segments of the mitochondrial DNA sequence which code for a single polypeptide sequence of cytochrome b. In each segment mutations which modify the polypeptide sequence can occur leading to the loss (BOX mutants) or to a modification (drug resistant mutants) of the enzyme activity.Chercheur qualifié du Fonds National de la Recherche Scientifique     

Non-random association of human acrocentric chromosomes

Summary The association of human acrocentric chromosomes was found to be nonrandom using fluorescence technic.The occurrence of chromosomes D13-15 in Robertsonian rearrangement has been shown to be non-random (Hecht et al., 1968). Ohno et al. (1961) have postulated these rearrangements might be related to acrocentric association. In two studies (Nakagame, 1969; Shaw et al., 1969), however, a random distribution of the D group chromosomes in association was found by autoradiography in lymphocytes. The recently described fluorescence technic (Caspersson et al., 1971) permits a reliable identification of each of the 23 pairs and we would like to report the results of our re-examination of acrocentric association by this method.

---

Zusammenfassung Die Assoziation menschlicher akrozentrischer Chromosomen hat sich als nicht zufällig erwiesen. Es wurde die Fluorescenztechnik benutzt.

     

Superinfection exclusion and changes in cellular transport processes in phage infected Salmonella typhimurium.

Summary The changes induced by bacteriophage P22 in the cellular transport process(es) of the host Salmonella typhimurium (Taneja et al., 1975; Khandekar et al., 1975; Bandyopadhyay and Chakravorty, 1976) involve interactions between the superinfection exclusion system of the resident prophage and the C immunity region of the superinfecting phage. The sieA gene of the prophage interferes with the changes in the cellular transport process induced by the superinfecting phage. However, if the superinfecting phage carries active C ₁ and C ₂ genes of the superinfecting phage seem to be expressed in the sie A⁺ lysogen.     

Development of a tissue cell culture bioassay for identifying mechanisms of inhibition of settlement of barnacle and tunicate larvae by surface‐colonizing marine bacteria

A marine bacterium D2 (CCUG 26757) isolated from a tunicate Ciona intestinalis specimen produced a low molecular weight component which inhibits barnacle and tunicate larvae and prevents their settlement on solid surfaces (Holmström et al., 1992). In order to perform chemical and structural analyses of the component independent of season, a bioassay, complementary to tests with invertebrate larvae was developed. This bioassay is based on tissue cell culture techniques and growth of the AGS cell line. It was previously shown that the toxic component is a stationary phase released product, which is heat stable and less than 500 Dalton in size (Holmström et al., 1992). Furthermore, it is not a peptide or a protein and metaperiodate treatment increases its toxicity to larvae indicating that it binds to or contains carbohydrate moieties. In this study, these results were confirmed by using the anchorage dependent human gastric adenocarcinoma (AGS) cell line as a bioassay. Fractionation of the D2 supernatant on a Sephadex G‐200 column and addition of different size fractions to the AGS tissue culture cells, showed that both a low and a high molecular weight fraction inhibited cell growth. Exposure of Balanus amphitrite and C. intestinalis larvae to the same fractions, showed that the low molecular weight fraction that inhibited growth of the cells corresponds to the component that inhibited larvae of both organisms. The high molecular weight fraction was found to inhibit larvae of B. amphitrite.     

Studies on autoimmune encephalomyelitis in the guinea pig--III. A comparative study of two autoantigens of central nervous system myelin.

Abstract— A new CNS myelin autoantigen(s) (referred to as M2), different from the encephalitogenic basic protein (BP), can be detected with guinea-pig demyelinating and complement fixing (CF) sera raised against guinea pig CNS tissue or myelin (Lebar et al., 1976). M2 and BP were present in mouse, rat, rabbit, bovine and human CNS tissues when tested with guinea-pig homologous specific antisera; they were not present in non-CNS tissues. Both autoantigens were also detected in newborn guinea-pig myelin and myelin-like fractions. The CF activity of myelin with demyelinating (anti-M2) sera was not altered by trypsin; however, absorption experiments showed that M2 was partly trypsin sensitive. Both antibodies against the trypsin sensitive and the trypsin resistant determinants of M2 were demyelinating. Both determinants of M2 were preselit in mouse, rat, rabbit, bovine‘and human CNS tissues and in guinea-pig newborn myelin. CF BP activity of myelin was partially or even totally abolished by trypsin, but the persistent encephali-togenicity of trypsin-treated myelin could be attributed to non-CF encephalitogenic peptides from BP. In accordance with recent work our results tend to support an inner localization of BP in myelin; M2, on the other hand, would be a surface antigen(s).     

Restriction enzyme map for streptomycete plasmid pUC3

A restriction enzyme map for the streptomycete plasmid pUC3 was constructed for the enzymes XhoI, EcoRI, HindIII, PstI, BamH-I, and BglII. The plasmid was isolated from Streptomyces sp. 3022a which produces chloramphenicol and has been referred to as S. venezuelae (Bewick et al., 1976 and Bewick and Williams, 1977, Microbios, 19, 27–35).     

Yeast ribosomal proteins. VIII. Isolation of two proteins and sequence characterization of twenty-four proteins from cytoplasmic ribosomes

Summary Two proteins, YL41 and YL43, were isolated from 80S ribosomes of Saccharomyces cerevisiae by filtration through a Sephacryl S-200 column and by chromatography on a column of carboxymethylcellulose. Their amino acid compositions are presented. Twenty-four proteins including these two proteins were subjected to sequence analyses by automated Edman degradation. Amino-terminal amino acid sequences were determined for 17 proteins, YS3, YS9, YS23, YS24, YS29, YL6, YL8, YL11, YL15, YL17, YL23, YL28, YL33, YL37, YL39, YL41, and YL43. YL41, which has a 72.7% lysine and arginine content, was found to be particular to eukaryotic ribosomes. The aminotermini of another seven proteins, YS2, YS5, YS8, YS12, YS13, YS20, and YS27, were suggested to be blocked.Comparison of the amino-terminal sequences with all other ribosomal protein sequences so far available indicates that YS9 shows sequence homology to rat liver ribosomal protein S8 (Wittmann-Liebold et al. 1979).     

Development of a photosynthesis model with an emphasis on ecological applications

Summary Photosynthesis was measured in leaves ofPhaseolus vulgaris and analyzed according to the set of equations outlined previously by Tenhunen et al. (1976).     

Production of presumptive humoral haematopoietic regulators in canine cyclic haematopoiesis

Abstract. Conditioned media (CM) were prepared according to previously published techniques from the bone marrow of dogs with cyclic haematopoiesis (CH). CM prepared from day 9 marrows inhibited mouse bone marrow CFU-s proliferation rate while CM from day 10 marrows were stimulatory and also contained an erythroid stimulating factor which appeared to be erythropoietin. In addition a highly significant trend from CM containing CFU-s inhibitory materials to media with CFU-s stimulatory activity was observed through cycles day 1 to 8. These studies further support the concept that CH is due to a defect in factors controlling stem cell proliferation and suggest that a major event occurs in CH dog marrow on days 9 and/or 10 of the cycle. Bone marrow transplantation studies (Dale & Graw, 1974; Weiden et al., 1974; Jones et al., 1975b) have indicated that canine cyclic haematopoiesis (CH) is probably due to a disorder in the multipotential stem cells. Morphological evidence (Scott et al., 1973) and the almost synchronous cycling of CFU-e, CFU-c and diffusion chamber progenitor cells (DCPC) (DUM et al., 1977, 1978a, b) lend support to such a theory. However, efforts to identify the mechanisms controlliig multipotential stem cell proliferation in dogs have been handicapped by the lack of suitable techniques to study these cells in the canine. Recently, Wright and co-workers (Wright & Lord, 1978, 1979; Wright et al., 1979; Lord et al., 1979), on the basis of previous observations (Frindel et al., 1976; Frindel & Guigon, 1977), described the preparation of species non-specific, bone marrow conditioned media (CM) which are capable of influencing the proliferation rate of murine colony forming units-spleen (CFU-s). The studies now reported were designed to determine if CM prepared from canine CH marrow would influence the proliferation rate of murine bone marrow CFU-s. The results indicate that a major event, possibly related to the in vivo control of stem cell proliferation in dogs with CH, occurs on days 9–10 of the cycle; day 1 being the first day when the peripheral blood neutrophil count falls below-1600 mm³.