The transcriptional landscape of the mammalian genome

This study describes comprehensive polling of transcription start and termination sites and analysis of previously unidentified full-length complementary DNAs derived from the mouse genome. We identify the 5' and 3' boundaries of 181,047 transcripts with extensive variation in transcripts arising from alternative promoter usage, splicing, and polyadenylation. There are 16,247 new mouse protein-coding transcripts, including 5154 encoding previously unidentified proteins. Genomic mapping of the transcriptome reveals transcriptional forests, with overlapping transcription on both strands, separated by deserts in which few transcripts are observed. The data provide a comprehensive platform for the comparative analysis of mammalian transcriptional regulation in differentiation and development.

Analysis of the mouse transcriptome based on functional annotation of 60,770 full-length cDNAs

Only a small proportion of the mouse genome is transcribed into mature messenger RNA transcripts. There is an international collaborative effort to identify all full-length mRNA transcripts from the mouse, and to ensure that each is represented in a physical collection of clones. Here we report the manual annotation of 60,770 full-length mouse complementary DNA sequences. These are clustered into 33,409 'transcriptional units', contributing 90.1% of a newly established mouse transcriptome database. Of these transcriptional units, 4,258 are new protein-coding and 11,665 are new non-coding messages, indicating that non-coding RNA is a major component of the transcriptome. 41% of all transcriptional units showed evidence of alternative splicing. In protein-coding transcripts, 79% of splice variations altered the protein product. Whole-transcriptome analyses resulted in the identification of 2,431 sense-antisense pairs. The present work, completely supported by physical clones, provides the most comprehensive survey of a mammalian transcriptome so far, and is a valuable resource for functional genomics.

Human pigmentation genes: identification, structure and consequences of polymorphic variation

The synthesis of the visible pigment melanin by the melanocyte cell is the basis of the human pigmentary system, those genes directing the formation, transport and distribution of the specialised melanosome organelle in which melanin accumulates can legitimately be called pigmentation genes. The genes involved in this process have been identified through comparative genomic studies of mouse coat colour mutations and by the molecular characterisation of human hypopigmentary genetic diseases such as OCA1 and OCA2. The melanocyte responds to the peptide hormones alpha-MSH or ACTH through the MC1R G-protein coupled receptor to stimulate melanin production through induced maturation or switching of melanin type. The pheomelanosome, containing the key enzyme of the pathway tyrosinase, produces light red/yellowish melanin, whereas the eumelanosome produces darker melanins via induction of additional TYRP1, TYRP2, SILV enzymes, and the P-protein. Intramelanosomal pH governed by the P-protein may act as a critical determinant of tyrosinase enzyme activity to control the initial step in melanin synthesis or TYRP complex formation to facilitate melanogenesis and melanosomal maturation. The search for genetic variation in these candidate human pigmentation genes in various human populations has revealed high levels of polymorphism in the MC1R locus, with over 30 variant alleles so far identified. Functional correlation of MC1R alleles with skin and hair colour provides evidence that this receptor molecule is a principle component underlying normal human pigment variation.

A large family of endosome-localized proteins related to sorting nexin 1

Sorting nexin 1 (SNX1), a peripheral membrane protein, has previously been shown to regulate the cell-surface expression of the human epidermal growth factor receptor [Kurten, Cadena and Gill (1996) Science 272, 1008-1010]. Searches of human expressed sequence tag databases with SNX1 revealed eleven related human cDNA sequences, termed SNX2 to SNX12, eight of them novel. Analysis of SNX1-related sequences in the Saccharomyces cerevisiae genome clearly shows a greatly expanded SNX family in humans in comparison with yeast. On the basis of the predicted protein sequences, all members of this family of hydrophilic molecules contain a conserved 70-110-residue Phox homology (PX) domain, referred to as the SNX-PX domain. Within the SNX family, subgroups were identified on the basis of the sequence similarities of the SNX-PX domain and the overall domain structure of each protein. The members of one subgroup, which includes human SNX1, SNX2, SNX4, SNX5 and SNX6 and the yeast Vps5p and YJL036W, all contain coiled-coil regions within their large C-terminal domains and are found distributed in both membrane and cytosolic fractions, typical of hydrophilic peripheral membrane proteins. Localization of the human SNX1 subgroup members in HeLa cells transfected with the full-length cDNA species revealed a similar intracellular distribution that in all cases overlapped substantially with the early endosome marker, early endosome autoantigen 1. The intracellular localization of deletion mutants and fusions with green fluorescent protein showed that the C-terminal regions of SNX1 and SNX5 are responsible for their endosomal localization. On the basis of these results, the functions of these SNX molecules are likely to be unique to endosomes, mediated in part by interactions with SNX-specific C-terminal sequences and membrane-associated determinants.

A dileucine motif targets E-cadherin to the basolateral cell surface in Madin-Darby canine kidney and LLC-PK1 epithelial cells

E-cadherin is a major adherens junction protein of epithelial cells, with a central role in cell-cell adhesion and cell polarity. Newly synthesized E-cadherin is targeted to the basolateral cell surface. We analyzed targeting information in the cytoplasmic tail of E-cadherin by utilizing chimeras of E-cadherin fused to the ectodomain of the interleukin-2alpha (IL-2alpha) receptor expressed in Madin-Darby canine kidney and LLC-PK(1) epithelial cells. Chimeras containing the full-length or membrane-proximal half of the E-cadherin cytoplasmic tail were correctly targeted to the basolateral domain. Sequence analysis of the membrane-proximal tail region revealed the presence of a highly conserved dileucine motif, which was analyzed as a putative targeting signal by mutagenesis. Elimination of this motif resulted in the loss of Tac/E-cadherin basolateral localization, pinpointing this dileucine signal as being both necessary and sufficient for basolateral targeting of E-cadherin. Truncation mutants unable to bind beta-catenin were correctly targeted, showing, contrary to current understanding, that beta-catenin is not required for basolateral trafficking. Our results also provide evidence that dileucine-mediated targeting is maintained in LLC-PK(1) cells despite the altered polarity of basolateral proteins with tyrosine-based signals in this cell line. These results provide the first direct insights into how E-cadherin is targeted to the basolateral membrane.

Characterisation of two distinct HKT1-like potassium transporters from Eucalyptus camaldulensis

Potassium is an essential macronutrient in higher plants. It plays an important physiological role in stoma movements, osmoregulation, enzyme activation and cell expansion. The demand for potassium can be substantial, especially when the plant concerned is a Eucalyptus tree in excess of 50 m tall. We have isolated two cDNAs, EcHKT1 and EcHKT2, from Eucalyptus camaldulensis (river red gum) which are expressed in leaves, stems and roots. These encode potassium transporter polypeptides with homology to the wheat K+-Na+ symporter, HKT1. EcHKT1 and EcHKT2 both complemented the K+-limited growth of an Escherichia coli K+-uptake-deficient triple mutant. EcHKT1 and EcHKT2 also mediated Na+ and K+ uptake when expressed in Xenopus oocytes. A comparison of the EcHKT1 and EcHKT2 sequences and their transport properties indicated that these cDNAs represent two K+ transporters with distinct functional characteristics. The functional and structural conservation between these two E. camaldulensis genes and the wheat HKT1 suggests that they play an important, albeit elusive, physiological role.

A DEF/GLO-like MADS-box gene from a gymnosperm: Pinus radiata contains an ortholog of angiosperm B class floral homeotic genes

The specification of floral organ identity during development depends on the function of a limited number of homeotic genes grouped into three classes: A, B, and C. Pairs of paralogous B class genes, such as DEF and GLO in Antirrhinum, and AP3 and PI in Arabidopsis, are required for establishing petal and stamen identity. To gain a better understanding of the evolutionary origin of petals and stamens, we have looked for orthologs of B class genes in conifers. Here we report cDNA cloning of PrDGL (Pinus radiata DEF/GLO-like gene) from radiata pine. We provide phylogenetic evidence that PrDGL is closely related to both DEF- and GLO-like genes of angiosperms, and is thus among the first putative orthologs of floral homeotic B function genes ever reported from a gymnosperm. Expression of PrDGL is restricted to the pollen strobili (male cones) and was not detected in female cones. PrDGL expression was first detected in emergent male cone primordia and persisted through the early stages of pollen cone bud differentiation. Based on the results of our phylogeny reconstructions and expression studies, we suggest that PrDGL could play a role in distinguishing between male (where expression is on) and female reproductive structures (where expression is off) in radiata pine. We speculate that this could be the general function of DEF/GLO-like genes in gymnosperms that may have been recruited for the distinction between stamens and carpels, the male and female reproductive organs of flowering plants, during the evolution of angiosperms out of gymnosperm-like ancestors.

Multiple interval mapping for quantitative trait loci

A new statistical method for mapping quantitative trait loci (QTL), called multiple interval mapping (MIM), is presented. It uses multiple marker intervals simultaneously to fit multiple putative QTL directly in the model for mapping QTL. The MIM model is based on Cockerham's model for interpreting genetic parameters and the method of maximum likelihood for estimating genetic parameters. With the MIM approach, the precision and power of QTL mapping could be improved. Also, epistasis between QTL, genotypic values of individuals, and heritabilities of quantitative traits can be readily estimated and analyzed. Using the MIM model, a stepwise selection procedure with likelihood ratio test statistic as a criterion is proposed to identify QTL. This MIM method was applied to a mapping data set of radiata pine on three traits: brown cone number, tree diameter, and branch quality scores. Based on the MIM result, seven, six, and five QTL were detected for the three traits, respectively. The detected QTL individually contributed from approximately 1 to 27% of the total genetic variation. Significant epistasis between four pairs of QTL in two traits was detected, and the four pairs of QTL contributed approximately 10.38 and 14.14% of the total genetic variation. The asymptotic variances of QTL positions and effects were also provided to construct the confidence intervals. The estimated heritabilities were 0.5606, 0.5226, and 0. 3630 for the three traits, respectively. With the estimated QTL effects and positions, the best strategy of marker-assisted selection for trait improvement for a specific purpose and requirement can be explored. The MIM FORTRAN program is available on the worldwide web (http://www.stat.sinica.edu.tw/chkao/).

A novel Golgi-localisation domain shared by a class of coiled-coil peripheral membrane proteins

The mechanism by which peripheral membrane proteins are targeted to the cytoplasmic face of the Golgi apparatus is poorly understood. Previously, we have identified a carboxy-terminal domain of the trans-Golgi-network (TGN) protein p230 that is responsible for Golgi localisation [1]. Here, we report the identification of a similar Golgi-localisation domain (GLD, also termed the 'GRIP' domain - see the paper by Munro and Nichols elsewhere in this issue) in a family of putative peripheral membrane proteins from lower and higher eucaryotes. The majority of family members have a domain structure similar to that of p230, with extensive coiled-coil regions (>80%) and the potential GLD located in a non-coiled-coil domain at the carboxyl terminus. Previously reported proteins in this family include human golgin-97 and Saccharomyces cerevisiae Imh1p. By constructing chimeric cDNAs encoding carboxy-terminal regions of these family members fused to green fluorescent protein (GFP), we have directly demonstrated that the GLD of p230, golgin-97, the newly identified human protein GCC1p and yeast Imh1p functions as a Golgi-targeting domain in transfected mammalian cells. Site-directed mutagenesis of the GLDs identified two conserved aromatic residues that are critical for the function of this targeting domain. Endogenous p230 was displaced from the Golgi membranes in transfected cells expressing high levels of GFP fused to the GLD of either p230 or golgin-97, indicating that different GLDs interact with similar membrane determinants. Thus, we have identified a family of coiled-coil proteins that share a domain shown to be sufficient for the localisation of peripheral membrane proteins to the Golgi apparatus.

Eucalypt MADS-box genes expressed in developing flowers

Three MADS-box genes were identified from a cDNA library derived from young flowers of Eucalyptus grandis W. Hill ex Maiden. The three egm genes are single-copy genes and are expressed almost exclusively in flowers. The egm1 and egm3 genes shared strongest homology with other plant MADS-box genes, which mediate between the floral meristem and the organ-identity genes. The egm3 gene was also expressed strongly in the receptacle or floral tube, which surrounds the carpels in the eucalypt flower and bears the sepals, petals, and numerous stamens. There appeared to be a group of genes in eucalypts with strong homology with the 3' region of the egm1 gene. The egm2 gene was expressed in eucalypt petals and stamens and was most homologous to MADS-box genes, which belong to the globosa group of genes, which regulate organogenesis of the second and third floral whorls. The possible role of these three genes in eucalypt floral development is discussed.

NEEDLY, a Pinus radiata ortholog of FLORICAULA/LEAFY genes, expressed in both reproductive and vegetative meristems

The LEAFY/FLORICAULA genes from Arabidopsis and Antirrhinum are necessary for normal flower development and play a key role in diverse angiosperm species. A homologue of these flower meristem-identity genes, NEEDLY (NLY), has been identified in Pinus radiata. Although the NLY protein shares extensive sequence similarity with its angiosperm counterparts, it is lacking the proline-rich and acidic motifs thought to function as transcriptional activation domains. NLY already is expressed during vegetative development at least 5 years before the transition to the reproductive phase. Expression of NLY in transgenic Arabidopsis promotes floral fate, demonstrating that, despite its sequence divergence, NLY encodes a functional ortholog of the FLORICAULA/LEAFY genes of angiosperms. Expression of the LFY::NLY transgene can largely complement the defects in flower development caused by a severe lfy allele.

Family of MADS-Box genes expressed early in male and female reproductive structures of monterey pine

Three MADS-box genes isolated from Monterey pine (Pinus radiata), PrMADS1, PrMADS2, and PrMADS3, are orthologs to members of the AGL2 and AGL6 gene subfamilies in Arabidopsis. These genes were expressed during early stages of pine shoot development in differentiating seed- and pollen-cone buds. Their transcripts were found within a group of cells that formed ovuliferous scale and microsporophyll primordia. Expression of PrMADS3 was also detected in a group of cells giving rise to needle primordia within differentiated vegetative buds, and in needle primordia.

Two pine endo-beta-1,4-glucanases are associated with rapidly growing reproductive structures

Two cDNA clones encoding endo-beta-1,4-glucanases (EGases) were isolated from a radiata pine (Pinus radiata) cDNA library prepared from immature female strobili. The cDNAs PrCel1 (inus adiata cellulase ) and PrCel2 encode proteins 509 and 515 amino acids in length, respectively, including putative signal peptides. Both proteins contain domains conserved in plant and bacterial EGases. The proteins PRCEL1 and PRCEL2 showed strong similarity to each other (76% amino acid identity), and higher similarity to TPP18 (73 and 67%, respectively), an EGase cloned from tomato (Lycopersicon esculentum) pistils, than to any other reported EGases. Northern-blot analyses indicated that both genes displayed a similar pattern of expression. The only significant difference was in the level of expression. In situ hybridizations were used to demonstrate that, within differentiating pine reproductive structures, PrCel1 expression was greatest in microsporangia in pollen strobili and near the developing ovule in the seed strobili. Expression was also found in vegetative tissues, especially in regions experiencing cell elongation, such as the elongating region of root tips. Both proteins have an ability to degrade carboxymethylcellulose in vitro. Genomic-blot analysis indicated the presence of a family of EGase genes in the radiata pine genome, and that PrCel1 and PrCel2 are transcribed from distinct one-copy genes.

Signal-mediated sorting of membrane proteins between the endoplasmic reticulum and the golgi apparatus

Each organelle of the secretory pathway is required to selectively allow transit of newly synthesized secretory and plasma membrane proteins and also to maintain a unique set of resident proteins that define its structural and functional properties. In the case of the endoplasmic reticulum (ER), residency is achieved in two ways: (a) prevention of residents from entering newly forming transport vesicles and (b) retrieval of those residents that escape. The latter mechanism is directed by discrete retrieval motifs: Soluble proteins have a H/KDEL sequence at their carboxy-terminus; membrane proteins have a dibasic motif, either di-lysine or di-arginine, located close to the terminus of their cytoplasmic domain. Recently it was found that di-lysine motifs bind the complex of cytosolic coat proteins, COP I, and that this interaction functions in the retrieval of proteins from the Golgi to the ER. Also discussed are the potential roles this interaction may have in vesicular trafficking.

Post-translational modifications distinguish cell surface from Golgi-retained beta 1,4 galactosyltransferase molecules. Golgi localization involves active retention

beta 1,4 Galactosyltransferase (GalT) is a membrane-bound enzyme localized predominantly to the trans-Golgi cisternae. Our previous studies have shown that the transmembrane domain of bovine GalT plays a critical role in Golgi localization (Teasdale, R.D., D'Agostaro, G. and Gleeson, P.A., J. Biol. Chem., 267, 4084-4096, 1992). Here we have compared the localization and post-translational modifications of full-length bovine GalT with a GalT/hybrid molecule where the transmembrane domain of GalT was replaced with that of the transferrin receptor. GalT/hybrid molecules were expressed on the surface of transfected cells; however, differences were observed in the distribution of the hybrid molecules between transfected COS and murine L cells. In transfected COS cells, the GalT/hybrid protein was expressed efficiently at the cell surface, with little Golgi-localized material, whereas in stable murine L cells, which expressed lower levels of the construct, hybrid molecules were detected both at the cell surface and within the Golgi apparatus. Expression of the GalT constructs in either COS or L cells produced two glycoprotein products which differed in molecular mass by 7 kDa. The difference in size between the two products is due to post-translational modifications which are inhibited by brefeldin A and are therefore likely to occur in the trans-Golgi network (TGN). Very little of the high-molecular-weight species was detected for full-length GalT, whereas it was a major product for the GalT/hybrid protein. Only the higher molecular weight species was expressed at the cell surface. Thus, this additional 7 kDa post-translational modification distinguishes molecules retained within the Golgi apparatus (lower M(r) species) from those transported through the TGN to the cell surface. These studies indicate that (i) the level of expression influences the intracellular distribution of GalT/hybrid molecules and (ii) the localization of full-length GalT involves active retention within the Golgi stack, and not retrieval from later compartments. After treatment of membrane preparations from stable L cell clones with a heterobifunctional cross-linking agent, full-length bovine GalT molecules were found almost exclusively as high-molecular-weight aggregates, suggesting that GalT exists as an oligomer or aggregate. This ability to oligomerize may be a requirement for Golgi retention.

Targeting of proteins to the Golgi apparatus

The Golgi apparatus maintains a highly organized structure in spite of the intense membrane traffic which flows into and out of this organelle. Resident Golgi proteins must have localization signals to ensure that they are targeted to the correct Golgi compartment and not swept further along the secretory pathway. There are a number of distinct groups of Golgi membrane proteins, including glycosyltransferases, recycling trans-Golgi network proteins, peripheral membrane proteins, receptors and viral glycoproteins. Recent studies indicate that there are a number of different Golgi localization signals and mechanisms for retaining proteins to the Golgi apparatus. This review focuses on the current knowledge in this field.

Producing a P1 bacteriophage library from pine: isolation and cloning of very high molecular weight DNA

We have generated a genomic P1 bacteriophage library using Monterey pine (Pinus radiata) DNA. We first developed a method for isolating from pine tissue the very high molecular weight DNA necessary for the preparation of libraries requiring large inserts. The method involves protoplasting the cells, isolating nuclei and lysis in a high concentration of detergent. Fragments of greater than two megabases in size are produced in solution. Modifications introduced to the protocol for library preparation and for P1 plasmid isolation are described.

The signal for Golgi retention of bovine beta 1,4-galactosyltransferase is in the transmembrane domain

The expression and localization of bovine beta 1,4-galactosyltransferase (Gal T) has been studied in mammalian cells transfected with Gal T cDNA constructs, and the role of the amino-terminal domains of Gal T in Golgi localization examined. Here we demonstrate that the transmembrane (signal/anchor) domain of bovine Gal T contains a positive Golgi retention signal. Bovine Gal T was characterized in transfected cells with anti-bovine Gal T antibodies, affinity-purified from a rabbit antiserum using a bacterial recombinant fusion protein. These affinity-purified antibodies recognized native bovine Gal T and showed minimum cross-reactivity with Gal T from non-bovine sources. Bovine Gal T cDNA was expressed, as active enzyme, transiently in COS-1 cells and stably in murine L cells, and the product was shown to be localized to the Golgi complex by immunofluorescence using the polypeptide-specific antibodies. A low level of surface bovine Gal T was also detected in the transfected L cells by flow cytometry. The removal of 18 of the 24 amino acids from the cytoplasmic domain of bovine Gal T did not alter the Golgi localization of the product transiently expressed in COS-1 cells or stably expressed in L cells. Both the full-length bovine Gal T and the cytoplasmic domain deletion mutant were N-glycosylated in the transfected L cells, indicating both proteins have the correct N(in)/C(out) membrane orientation. Deletion of both the cytoplasmic and signal/anchor domains of bovine Gal T and incorporation of a cleavable signal sequence resulted in a truncated soluble bovine Gal T that was rapidly secreted (within 1 h) from transfected COS-1 cells. Replacement of the signal/anchor domain of bovine Gal T with the signal/anchor domain of the human transferrin receptor resulted in the transport of the hybrid molecule to the cell surface of transfected COS-1 cells. Furthermore, a hybrid construct containing the signal/anchor domain of Gal T with ovalbumin was efficiently retained in the Golgi complex, whereas ovalbumin anchored to the membrane by the transferrin receptor signal/anchor was expressed at the cell surface of transfected COS-1 cells. Overall, these studies show that the hydrophobic, signal/anchor domain of Gal T is both necessary and sufficient for Golgi localization.

Boron deficiency in cultured pine cells : quantitative studies of the interaction with ca and mg

A pronounced interaction between calcium, magnesium, and boron was found in growth studies with Pinus radiata cell cultures. Quantitative isoactivity data for the interaction was analyzed in terms of selected simple and plausible theoretical models. The data was found to be consistent with a model in which a critical acceptor molecule is activated only by binding both Ca and B at separate sites; Mg competitively displaces Ca to inactivate the acceptor. It was found that B is, surprisingly, not bound strongly (K(diss) = 450 +/- 80 micromolar) and that the affinity for Ca is two orders of magnitude stronger than for Mg. Therefore only a small proportion of the acceptor will be boronated under natural conditions. Moderate levels of mannitol were found to aggravate B deficiency due to its effective removal by direct chemical complexation. At higher concentrations of mannitol (or other sugars), where osmotic contribution is significant, little B was needed to overcome growth inhibition-a result consistent with B having a primary role in cell wall biosynthesis.

Mineral Nutrient Requirements of a Loblolly Pine (Pinus taeda) Cell Suspension Culture : Evaluation of a Medium Formulated from Seed Composition Data

The mineral nutrient requirements of Pinus taeda cells were explored using quantitative cell culture growth measurements. An appraisal was thereby made of the critical features of a novel and successful medium which was developed specifically for this gymnosperm using chemical composition data for developing seeds, and characterized by generally high concentration of all micronutrients, high magnesium, and low calcium. The high magnesium concentration was found not to be detrimental and possibly beneficial whereas the calcium level bordered on a deficiency threshold. Within the microelements high iodide was found to be essential, as was a higher borate level than is present in media developed for angiosperms. High zinc concentrations were also beneficial, with normal levels permitting slower but nevertheless healthy growth. An improved medium was thereby formulated which was stress-free and exhibited broader genotype specificity. This new formulation has proved very successful in maintaining long-term growth of highly uniform and apparently meristematic suspension cultures of Pinus radiata.

Substrate aggregation and cooperative enzyme kinetics: consideration of enzyme access with large aggregates

Consideration was given to a system in which an enzyme substrate indefinitely self-associates according to a general model with f sites of aggregation and a single intrinsic binding constant k. Where enzyme attack occurs at random points on the substrate surface, surface regions will be obscured from enzyme access by aggregate formation. The significance of reduced surface access for the infinite array of different size aggregates which co-exist was explored through use of a reacted site probability function, PA: the proportion of total substrate surface which is accessible to the enzyme was estimated as the fraction of total sites for aggregation which are unoccupied. The effective substrate concentration was thereby specified in terms of total substrate concentration (mA) by the simple expression mA = (1 - PA)mA. Plots of simulated v versus mA data were examined for a Michaelis-Menten enzyme of maximal velocity Vm and Michaelis constant Km to reveal deviations from expected enzyme behaviour; corresponding Hofstee (v/mA versus v) plots were found to be convex to the v axis as is characteristic of a negatively cooperative enzyme. As self-association is known to occur widely with large or small molecules in solution, the experimenter should be aware of the potential for these phenomena in kinetic studies to produce pseudo-allosteric effects, or to mask true allosteric behaviour.

Interpretation of the kinetics of consecutive enzyme-catalysed reactions: studies on the arginase-ornithine carbamoyltransferase system

A method that permits the use of measurements on the concentration of the intermediate in a coupled enzymic assay in determining the presence or absence of an interaction between the enzymes is presented. The method is shown to be closely analogous to a previously formulated procedure involving the determination of the rate of production of the final product of such a sequence and is shown to be applicable regardless of the complexity of the operative kinetic mechanisms, provided it may be assumed that all enzyme-substrate complexes are in the steady-state. Kinetic results obtained with the arginase-ornithine carbamoyltransferase couple, in which the intermediate ornithine is monitored, are examined in these terms to conclude that no heterogeneous association is operative between the enzymes.

Studies of macromolecular heterogeneous associations involving cross-linking: a re-examination of the ovalbumin-lysozyme system

A system is considered in which a multivalent acceptor interacts with a bivalent ligand in solution to form an array of complexes via multiple binding and cross-linking reactions. With the use of reacted site probability functions expressions are derived in terms of a site binding constant which are of potential use in the interpretation of sedimentation equilibrium and binding results obtained with such systems. Their potential use is explored in relation to results obtained on the interacting ovalbumin-lysozyme system at pH 6.80, ionic strength 0.02. A comparison is made of this interpretation with that based on an interaction pattern involving only multiple binding of ligand in the absence of cross-linking effects. While both interpretations quantitatively describe certain results, it is shown, by invoking further experimental observations on apparent weight-average molecular weight and precipitation behavior, that the more favored interpretation is that involving the operation of a spectrum of forces leading to a large array of ovalbumin-lysozyme complexes, including those of the cross-linked type. It is stressed that the particular ovalbumin-lysozyme system is but one example of interaction between oppositely charged macromolecules and therefore that the derived equations may find wider application to such systems and those known to involve more specific cross-linking interactions.