The expressed class II alpha-chain genes of the marsupial major histocompatibility complex belong to eutherian mammal gene families

The major histocompatibility complex (Mhc) is a multigene family found in vertebrates. Mhc genes code for heterodimeric cell-surface molecules involved in presentation of peptides to T-lymphocytes. There are two classes of Mhc, and in eutherian mammals four main families of class II genes have been recognized; DR, DQ, DP, and DN/DO. Each class II family contains genes that code for one or more alpha and beta chains. Do the class II genes of marsupial mammals belong to any of these eutherian mammal class II families? The results to date are conflicting. The expressed class II beta-chain genes could not be satisfactorily assigned to any eutherian class II gene family and were designated as new gene families, while, conversely, a partial sequence of an expressed alpha-chain gene was clearly very similar to the DNA gene of eutherian mammals. The aim of this study was to conduct a more thorough analysis of the alpha-chain genes in a marsupial by obtaining full-length sequences of all the expressed alpha-chain genes in the red-necked wallaby, Macropus rufogriseus. Two class II alpha-chain genes were isolated from a spleen-derived cDNA library, and both have the potential to code for fully functional MHC molecules. Phylogenetic analysis indicated they belonged to previously identified eutherian class II families and are designated as Maru-DRA and Maru-DNA. Northern blot data indicated processed transcript sizes of approximately 1.6 kb for Maru-DRA and approximately 2.5 kb for Maru-DNA and that the latter was expressed at a lower level than the former. The phylogeny shows that the DR, DQ, DP, and DN/DO gene families diverged prior to the divergence of the marsupial and eutherian mammal lineages.

Major histocompatibility complex class I genes of the coelacanth Latimeria chalumnae

The coelacanth fish Latimeria chalumnae is the sole surviving species of a phylogenetic lineage that was founded more than 400 million years ago and that has changed morphologically very little since that time. Little is known about the molecular evolution of this "living fossil," considered by some taxonomists to be the closest living relative of tetrapods. Here we describe the isolation and characterization of L. chalumnae major histocompatibility complex (MHC) class I genes. The exon-intron organization of these genes is the same as that of their mammalian counterparts. The genes fall into four families, which we designate Lach-UA through Lach-UD. There are multiple loci in all of the families. Genes of the first two families are transcribed. The Lach-UA family bears the characteristics of functional, polymorphic class I genes; the other three families may be represented by nonclassical genes. All the Lach loci arose by duplication from an ancestral gene after the foundation of the coelacanth lineage. Intergenic variation is highest at positions corresponding to the mammalian peptide-binding region. The closest relatives of the Lach genes among the MHC genes sequenced thus far are those of the amphibian Xenopus.

Organization of Mhc class II B genes in the zebrafish (Brachydanio rerio)

Using three genomic phage libraries, we isolated 26 clones from the zebrafish MHC class II B region. By restriction mapping, the clones could be arranged into six clusters, most clusters consisting of several overlapping clones. The combined clusters cover a total of 161 kb of the zebrafish class II region. Hybridization with specific probes demonstrated the presence in the clusters of two class II A and six class II B genes. Sequencing of the B genes revealed that they represented six different families of class II loci. Only two of the class II B and one of the class II A genes are complete; the others are truncated pseudogenes. Only one of the class II B loci shows extensive restriction fragment length polymorphism. This is also the only locus found to be transcribed in organs with large numbers of lymphoid or myeloid cells. The zebrafish class II genes have promoter regions with sequence elements found previously in mammalian genes and known to be involved in regulation of expression. The exon-intron organization of the zebrafish class II genes is similar to that of the mammalian genes, but the introns are characteristically short, ranging in length from 74 to 362 bp. The distances between A and B genes in a given pair are also short, but the distances between B genes are as long as or longer than those between mammalian class II B genes. All of the zebrafish class II B genes appear to have arisen by duplication and diversification of a single ancestral B gene after the separation of bony fishes from other vertebrate taxa.

Alu elements of the primate major histocompatibility complex

The chromosomal region constituting the major histocompatibility complex (MHC) has undergone complex evolution that is often difficult to decipher. An important aid in the elucidation of the MHC evolution is the presence of Alu elements (repeats) which serve as markers for tracing chromosomal rearrangements. As the first step toward the establishment of sets of evolutionary markers for the MHC, Alu elements present in selected MHC haplotypes of the human species, the gorilla, and the chimpanzee were identified. Restriction fragments of cosmid clones from the libraries of the three species were hybridized with Alu-specific probes, Alu elements were amplified by the polymerase chain reaction, and the amplification products were sequenced. In some cases, sequences of the regions flanking the Alu elements were also obtained. Altogether, 31 new Alu elements were identified, representing six Alu subfamilies. The average density of Alu elements in the MHC is one element per four kilobases (kb) of sequence. Alu elements have apparently been inserted steadily into the MHC over the last 65 million years (my). On average, one Alu element is inserted into the primate MHC every 4 my. Analysis of the human DR3 haplotype supports its origin by duplication from an ancestral haplotype consisting of DRB1 and DRB2 genes. The sharing of an old Alu element by the DRB1 and DRB2 genes, in turn, supports their divergence from a common ancestor more than 55 my ago.

Protoplasts from Phaseolus coccineus L. pulvinar motor cells show circadian volume oscillations

The circadian movement of the lamina of the primary leaf of Phaseolus coccineus is mediated by circadian volume changes of the extensor and flexor cells in the upper and lower half of the laminar pulvinus. Isolated protoplasts from the extensor, flexor, and flank cells of the pulvinus showed a circadian volume rhythm with a period longer than 24 h. In the case of the flexor protoplasts, we found a period length of 28 h, which is similar to that of the pulvinar cells in situ. In the extensor protoplasts, the volume rhythm was synchronized with 14-h light/10-h dark cycles. The larger volume was correlated with the early hours in the light period and the smaller volume with the dark period, as would be expected from the behavior of the extensor cells in situ.

Resolution of the HLA-DRB6 puzzle: a case of grafting a de novo-generated exon on an existing gene

HLA-DRB6, one of the human major histocompatibility complex genes, lacks exon 1, which normally codes for the leader and the first four amino acid residues of the mature protein. Because it also lacks the HLA promoter, it was surprising to find that the gene is transcribed at a low level in a chimpanzee B-lymphoblastoma cell line, in which the DRB6 homolog is truncated as in humans. The study designed to resolve the paradox has revealed that a retrovirus related to the mouse mammary tumor viruses was inserted into intron 1 of DRB6 > 23 million years ago. The insertion was either accompanied or followed by the deletion of exon 1 and the promoter region of DRB6. In the 3' long terminal repeat of the retrovirus, however, an open reading frame for a new exon arose, which codes for a sequence of mostly hydrophobic amino acid residues; the sequence could function as a leader for the truncated DRB6 gene. This new exon has a functional donor splice site at its 3' end, which enables it to be spliced in register with DRB6 exon 2. Upstream from the new exon is a promoter enabling transcription of the DRB6 gene. Besides providing an example of a de novo generation of an exon, the study suggests a potential mechanism for generating new genes through the replacement of old exons with newly generated ones.

Zebrafish Mhc class II alpha chain-encoding genes: polymorphism, expression, and function

Its small size and short generation time renders the zebrafish (Brachydanio rerio) an ideal vertebrate for immunological research involving large populations. A prerequisite for this is the identification of the molecules critical for an immune response in this species. In earlier studies, we cloned the zebrafish genes coding for the beta chains of the class I and class II major histocompatibility complex (Mhc) molecules. Here, we describe the cloning of the zebrafish alpha chain-encoding class II gene, which represents the first identification of a class II A gene in teleost fishes. The gene, which is less than 3 kilobases (kb) distant from one of the beta chain-encoding genes, is approximately 1.2 kb long and consists of four exons interrupted by very short (< 200 base pairs) introns. Its organization is similar to that of the mammalian class II A genes, but its sequence differs greatly from the sequence of the latter (36% sequence similarity). Among the most conserved parts is the promoter region, which contains X, Y, and TATA boxes with high sequence similarity to the corresponding mammalian boxes. The observed striking conservation of the promoter region suggests that the regulatory system of the class II genes was established more than 400 million years ago and has, principally, remained the same ever since. Like the DMA, but unlike all other mammalian class II A genes, the zebrafish gene codes for two cysteine residues which might potentially be involved in the formation of a disulfide bond in the alpha 1 domain. The primary transcript of the gene is 1196 nucleotides long and contains 708 nucleotides of coding sequence. The gene is expressed in tissues with a high content of lymphoid/myeloid cells (spleen, pronephros, hepatopancreas, and intestine). The analyzed genomic and cDNA sequences are probably derived from different loci (their overall sequence similarity in the coding region is 73% and their 3' untranslated regions are highly divergent from each other). The genes are apparently functional. Comparison of genes from different zebrafish populations reveals high exon 2 variability concentrated in positions coding for the putative peptide-binding region. Phylogenetic analysis suggests that the zebrafish class II A genes stem from a different ancestor than the mammalian class II A genes and the recently cloned shark class II A gene.

Conservative evolution of the Mbc-DP region in anthropoid primates

To determine the organization of the DP region in the Mbc of anthropoid primates, we constructed contig maps from cosmid clones of the chimpanzee and orangutan, representatives of the infraorder Catarrhini, as well as of the cotton-top tamarin, a representative of the infraorder Platyrrhini. We found the maps to be remarkably similar to each other and to the previously published map of the human DP region. In each of the four species, the DP region consists of four loci arranged in the same order (DPB2 . . . DPA2 . . . DPB1 . . . DPA1) and in the same transcriptional orientation (tail-to-tail). The regions in the four species are of approximately the same length and many of the restriction sites are shared between species. The inserts of most Alu elements, of a ribosomal protein pseudogene, and of an IgC epsilon-like pseudogene are found in corresponding positions in all four species. The data indicate that the human-type organization of the DP region was established before the divergence of the Catarrhini and Platyrrhini lines more than 37 million years ago and that it has remained principally intact since that time. This conservation of the DP region is in striking contrast to the evolutionary instability of certain other Mbc regions, in particular those occupied by the DRB or C4 and CYP21 loci. We interpret the stability of the DP region as an indication that the region is being phased out functionally.

Mhc-DRB genes of platyrrhine primates

The two infraorders of anthropoid primates, Platyrrhini (New World monkeys) and Catarrhini (Old World monkeys and the hominoids) are estimated to have diverged from a common ancestor 37 million years ago. The major histocompatibility complex class II DRB gene and haplotype polymorphism of the Catarrhini has been characterized in several recent studies. The present study was undertaken to obtain information on the DRB polymorphism of the Platyrrhini. Fifty-five complete exon 2 DRB sequences were obtained from six species of Platyrrhini representing both the Callitrichidae and the Cebidae families. Combined with the results of a parallel contig mapping study, our data indicate that at least three loci (DRB1*03, DRB3, and DRB5) are shared by the Catarrhini and the Platyrrhini. However, the three loci are occupied by functional genes in the former infraorder and mostly by pseudogenes in the latter. Instead of the pseudogenes, the Platyrrhini have evolved a new set of apparently functional genes-DRB11 and DRB*W12 through DRB*W19, which have thus far not been found in the Catarrhini. The DRB*W13, *W14, *W15, *W17, *W18, and *W19 genes seem to be restricted to the Cebidae family, whereas the DRB*W16 locus has so far been documented in the Callitrichidae family only. The DRB alleles of the cotton-top tamarin, and perhaps also those of the common marmoset (both members of the family Callitrichidae), are characterized by low nucleotide diversity, possibly indicating that they diverged from a common ancestral gene relatively recently.

Class I major histocompatibility complex genes of the red-necked Wallaby, Macropus rufogriseus

Marsupials are one of three main evolutionary lineages in mammals, the other two being the monotremes and the placental mammals. The marsupial and the placental lineages separated between 120 and 156 million years ago. In this communication, we provide the first molecular description of class I major histocompatibility complex (Mhc) genes in a representative of the marsupial lineage, the red-necked wallaby, Macropus rufogriseus. Three different, nearly full-length class I Mhc sequences were identified in the cDNA library prepared from spleen mRNA of a single wallaby. The three sequences identify at least two loci. Under the assumption that two of the identified sequences are alleles, we designate the three wallaby genes Maru-Mhc-UA*01, Maru-Mhc-UA*02, and Maru-Mhc-UB*01. The three Maru sequences share several codon deletions and insertions not found in the class I genes of placental mammals. Comparisons of genetic distances among the known class I genes suggest that the Maru genes arose from one ancestral element, whereas the class I genes of the placental mammals arose from another, different ancestral element. The absence of an identifiable defect in the three Maru sequences suggests that the genes from which they were derived are functional. Hence, as in placental mammals, there appear to be two functional class I Mhc loci in the marsupials as well.

Multiplication of Mhc-DRB5 loci in the orangutan: implications for the evolution of DRB haplotypes

The beta chain-encoding (B) class II genes of the primate major histocompatibility complex belong to several families. The DRB family of class II genes is distinguished by the occurrence of haplotype polymorphism--the existence of multiple chromosomal forms differing in length, gene number, and gene combinations, each form occurring at an appreciable frequency in the population. Some of the haplotypes, or fragments thereof, are shared by humans, chimpanzees, and gorillas. In an effort to follow the DRB haplotype polymorphism further back in time, we constructed DRB contig maps of the two chromosomes present in the orangutan cell line CP81. Two types of genes were found in the two haplotypes, Popy-DRB5 and Popy-DRB1*03, the former occurring in two copies and one gene fragment in each haplotype, so that the CP81 cell line contains four complete DRB5 genes and two DRB5 fragments altogether. Since the four genes are more closely related to one another than they are to other DRB5 genes, they must have arisen from a single ancestral copy by multiple duplications. At the same time, however, the two CP81 haplotypes differ considerably in their restriction enzyme sites and in the presence of Alu elements at different positions, indicating that they have been separated for a length of time that exceeds the lifespan of a primate species. Moreover, a segment of about 100 kilobase pairs is shared between the orangutan CP81-1 and the human HLA-DR2 haplotype. These findings indicate that part of the haplotype polymorphism may have persisted for more than 13 million years, which is the estimated time of human-orangutan divergence.

Different modes of Mhc evolution in primates

The human major histocompatibility complex (Mhc) is a chromosomal segment approximately 4 million bp long that contains > or = 84 genes. Some of these genes code for the class I and class II molecules, while the remaining genes code for complement components, cytochrome P450, tumor necrosis factor, and many other, unrelated proteins. We demonstrate on three examples (DP, C4-CYP21, and DRB) that different regions of the Mhc have different evolutionary histories. The organization of the DP region, which in humans contains four genes, was established in the ancestral Anthropoidea or earlier and has not changed since. The duplication that generated the two C4-CYP21 modules occurred in the ancestral Catarrhini or earlier, but the region has been undergoing periodic homogenizations via unequal crossing-over, which make paralogous genes in the same species more similar to each other than to orthologous genes of different species. The eight or nine genes of the DRB region were also generated in the ancestral Catarrhini, but the region has since been subject to frequent rearrangements, which generated various DRB haplotypes. Not only the alleles but, in part, also the haplotype polymorphism is evolving transspecifically. The DRB region of the Platyrrhini has an origin different from that of the Catarrhini. The picture emerging from these studies is that of both stability in some regions of the Mhc and tremendous evolutionary instability in other regions.

Trans-species origin of Mhc-DRB polymorphism in the chimpanzee

Trans-specific evolution of allelic polymorphism at the major histocompatibility complex loci has been demonstrated in a number of species. Estimating the substitution rates and the age of trans-specifically evolving alleles requires detailed information about the alleles in related species. We provide such information for the chimpanzee DRB genes. DNA fragments encompassing exon 2 were amplified in vitro from genomic DNA of ten chimpanzees. The nucleotide sequences were determined and their relationship to the human DRB alleles was evaluated. The alleles were classified according to their position in dendrograms and the presence of lineage-specific motifs. Twenty alleles were found at the expressed loci Patr-DRB1, -DRB3, -DRB4, -DRB5, and at the pseudogenes Patr-DRB6, -DRB7; of these, 13 are new alleles. Two other chimpanzee sequences were classified as members of a new lineage tentatively designated DRBX. Chimpanzee counterparts of HLA-DRB1*01 and *04 were not detected. The number of alleles found at individual loci indicates asymmetrical distribution of polymorphism between humans and chimpanzees. Estimations of intra-lineage divergence times suggest that the lineages are more than 30 million years old. Predictions of major chimpanzee DRB haplotypes are made.

Effects of tetraethylammoniumchloride (TEA), vanadate, and alkali ions on the lateral leaflet movement rhythm of Desmodium motorium (Houtt.) Merr

The period (approximately 3-5 min) of the ultradian rhythm of the lateral leaflet movement of Desmodium motorium is strongly lengthened (less than or equal to 30-40%) by the K+ channel blocker tetraethylammoniumchloride (20, 30, and 40 mM) and vanadate (0.5 and 1 mM), which is an effective inhibitor of the plasma membrane-bound H+ pump. The alkali ions K+, Na+, Rb+, and Cs+ (10-40 mM) shorten the period only slightly (less than or equal to 10-15%). Li+ (5-30 mM), however, increases the period of the leaflet rhythm drastically (less than or equal to 80%). We concluded that the plasmalemma-H(+)-ATPase-driven K+ transport through K+ channels is an essential component of the ultradian oscillator of Desmodium, as has been proposed for the circadian oscillator.

Shared polymorphism between gorilla and human major histocompatibility complex DRB loci

A high degree of polymorphism and high nucleotide diversity mark the functional genes of the major histocompatibility complex (Mbc). Alleles at the different Mbc loci can be classified into distinct lineages that are shared between species and, therefore, are presumed to have been founded before speciation. We have sequenced the most polymorphic part of 25 gorilla Mbc-DRB genes from six individuals. (The DRB genes code for the beta-polypeptide chain of the alpha beta heterodimer that constitutes one family of the class II MHC molecules.) Fifteen of the sequences identify new alleles at four DRB loci; each of the six gorillas was heterozygous at one of the loci at least. Thirteen of the alleles could be assigned to lineages identified previously; the remaining two alleles represent new lineages. All the major human DRB allelic lineages are now known to be shared with apes, and all must have originated before the human-gorilla-chimpanzee divergence more than six million years (my) ago. The presence of some of the gorilla and human lineages in Old World monkeys suggests that these lineages emerged before the divergence of apes and cercopithecids. We argue that the major allelic lineages at the DRB1 locus began to diverge shortly after the rounds of duplication that generated the different DRB loci now found in the hominoids and that this event occurred more than 30 my ago. Comparison of closely related gorilla DRB sequences indicates that polymorphism may be generated by several mechanisms: point mutations, slippage during DNA replication, and recombination. Deduced gene linkages provide evidence for transspecies evolution of haplotype polymorphism.

Major-histocompatibility-complex DRB genes of a New-World monkey, the cottontop tamarin (Saguinus oedipus)

The DRB region of the human and great-ape major histocompatibility complex displays not only gene but also haplotype polymorphism. The number of genes in the human DRB region can vary from one to four, and even greater variability exists among the DRB haplotypes of chimpanzees, gorillas, and orangutans. Accumulating evidence indicates that, like gene polymorphism, part of the haplotype polymorphism predates speciation. In an effort to determine when the gene haplotype polymorphisms emerged in the primate lineage, we sequenced three cDNA clones of the New-World monkey, the cottontop tamarin (Saguinus oedipus). We could identify two DRB loci in this species, one (Saoe-DRB1) occupied by apparently functional alleles (*0101 and *0102) which differ by only two nucleotide substitutions and the other (Saoe-DRB2) occupied by an apparent pseudogene. The Saoe-DRB2 gene contains an extra sequence derived from the 3' portion of exon 2 and placed 5' to this exon. This sequence contains a stop codon which makes the translation of the bulk of the Saoe-DRB2 gene unlikely. Preliminary Southern blot hybridization analysis with probes derived from these two genes suggests that both the DRB gene polymorphism and the haplotype polymorphism in the cottontop tamarin may be low. In most individuals the DRB region of this species probably consists of three genes. Comparisons of the Saoe-DRB sequences with those of other primates suggest that probably all of the DRB genes found until now in the Catarrhini were derived from a common ancestor after the separation of the Catarrhini and Platyrrhini lineages. The extant DRB gene and haplotype polymorphism may therefore have been founded in the mid-Oligocene some 33 Mya.

Shared class II MHC polymorphisms between humans and chimpanzees

To gain an insight into the evolution of the major histocompatibility complex alleles, three DRB and one DRA genes were isolated from chimpanzee cDNA libraries. The nucleotide sequences of the chimpanzee DRB (ChLA-DRB) genes were then compared with those of the available HLA-DRB alleles by constructing unrooted phylogenetic trees. All three ChLA-DRB genes were found to be more closely related to certain HLA-DRB alleles than unrelated HLA-DRB alleles are to each other. Since available evidence does not support the convergent evolution of MHC alleles, this result is consistent with the idea that closely related ChLA-DRB and HLA-DRB alleles are derived from common ancestral alleles, the existence of which predates the divergence of human and chimpanzee lineages. The predicted amino acid sequences of mature ChLA-DRA and HLA-DRA molecules differ by only one amino acid.

Isolation of functional extensor and flexor protoplasts fromPhaseolus coccineus L. pulvini: potassium induced swelling

Methods are described for the isolation of functional protoplasts from secondary pulvinus tissue (flexor and extensor) and from leaf mesophyll tissue of primary leaves ofPhaseolus coccineus L. Integrity of the protoplasts was shown by vital staining and their ability to evolve oxygen in the light. Extensor-cell protoplasts increased their volume for up to 60% upon addition of 10 mM KCl. This K(+)-induced swelling was accompanied by increased rates of proton extrusion.

Nucleotide sequences of chimpanzee MHC class I alleles: evidence for trans-species mode of evolution

To obtain an insight into the evolutionary origin of the major histocompatibility complex (MHC) class I polymorphism, a cDNA library was prepared from a heterozygous chimpanzee cell line expressing MHC class I molecules crossreacting with allele-specific HLA-A11 antibodies. The library was screened with human class I locus-specific DNA probes, and clones encoding both alleles at the A and B loci have been identified and sequenced. In addition, the sequences of two HLA-A11 subtypes differing by a single nucleotide substitution have been obtained. The comparison of chimpanzee and human sequences revealed a close similarity (up to 98.5%). The chimpanzee A locus alleles showed greatest similarity to the human HLA-A11/A3 family of alleles, one of them being very close to HLA-A11. Similarly, segments of the ChLA-B alleles displayed greatest similarity to certain HLA-B alleles. The calculated evolutionary branch point for the A11-like alleles is 7 x 10(6) to 9 x 10(6) years, whereas the other A locus alleles diverged between 12 x 10(6) and 17 x 10(6) years ago. Since the human and chimpanzee lineages separated 5 x 10(6) to 7 x 10(6) years ago, our data support the notion that during evolution, MHC alleles are transmitted from one species to the next.

Cell walls as reservoirs of potassium ions for reversible volume changes of pulvinar motor cells during rhythmic leaf movements

The laminar pulvinus of primary leaves of Phaseolus coccineus L. was investigated with respect to the total K(+) content, the apoplastic K(+) content, and the water potential of extensor and flexor sections in relation to the leaf positions in a circadian leaf-movement cycle, as well as the cation-exchange properties of isolated extensor- and flexor-cell walls. Turgid tissue showed a high total but low apoplastic K(+) content, shrunken tissue a low total but high apoplastic K(+) content. Thus, part of the K(+) transported into and out of the swelling or shrinking protoplasts is shuttled between the protoplasts and the surrounding walls, another part between different regions of the pulvinus. The K(+) fraction shuttled between protoplasts and walls was found to be 30-40% of the total transported K(+) fraction. Furthermore, 15-20% of the total K(+) content of the tissue is located in the apoplast when the apoplastic reservoir is filled, 5-10% when the apoplastic reservoir is depleted. The ion-exchange properties of walls of extensor and flexor cells appear identical in situ and in isolated preparations. The walls behave as cation exchangers of hhe weak-acid type with a strong dependence of the activity of fixed negative charges as well as of the K(+)-storing capacity on pH and [K(+)] of the equilibration solution. The high apoplastic K(+) contents of freshly cut tissues reflect the cation-storing capacity of the isolated walls. We suggest that K(+) ions of the Donnan free space are used for the reversible volume changes (mediating the leaf movement) mainly by an electrogenic proton pump which changes the pH and-or the [K(+)] in the water free space of the apoplast.

Nucleotide sequence analysis of class II genes borne by mouse t chromosomes

Five class IIH-2genes borne bytchromosomes were partially sequenced:borne by thetTuw10chromosome of EDY589 (H-2w2);andborne by thetTuw8chromosome of CR0437 (H-2w57), as well asandborne by thetTuw7chromosome of CR0435 (H-2w37). These genes are representatives of the three major groups of alleles found associated withtchromosomes. The sequenced part consisted of almost the entire exon 2 and the entire exon 3 coding for the first and the second domain of the β polypeptide chains, respectively. The sequence was compared with the published sequences ofAβandEβalleles borne, by non-tchromosomes. The comparison revealed that thet-associated alleles are no more similar to one another than they are to the corresponding genes present on non-tchromosomes in laboratory mice. This divergence in the nucleotide sequence among the class II genes is interpreted as evidence that thetcomplex is very old.

Mechanics of circadian pulvini movements in Phaseolus coccineus L. : Shape and arrangement of motor cells, micellation of motor cell walls, and bulk moduli of extensibility ([Formula: see text])

The circadian movement of the lamina of primary leaves of Phaseolus coccineus L. is mediated by antagonistic changes in the length of the extensor and flexor cells of the laminar pulvinus. The cortex of the pulvinus is a concentric structure composed of hexagonal disc-like cells, arranged in longitudinal rows around the central stele. Observations with polarization optics indicate that the cellulose microfibrils are oriented in a hoop-like fashion in the longitudinal walls of the motor cells. This micellation is the structural basis of the anisotropic properties of the cells: tangential sections of the extensor and flexor placed in hypotonic mannitol solutions showed changes only in length. As a consequence a linear correlation between length and volume was found in these sections. Based on the relationship between the water potential (which is changed by different concentrations of mannitol) and the relative volume of the sections and on the osmotic pressure at 50% incipient plasmolysis, osmotic diagrams were constructed for extensor and flexor tissues (cut during night position of the pulvinus). The bulk moduli of extensibility, [Formula: see text], were estimated from these diagrams. Under physiological conditions the [Formula: see text] values were rather low (in extensor tissue below 10 bar, in flexor tissue between 10 to 15 bar), indicating a high extensibility of the longitudinal walls of the motor cells. They are strongly dependent on the turgor pressure at the limits of the physiological pressure range.In well-watered plants, the water potentials of the extensor and flexor tissues were surprisingly low,-12 bar and-8 bar, respectively. This means that the cells in situ are by no means fully turgid. On the contrary, the cell volume in situ is similar to the volume at the point of incipient plasmolysis: the cell volumes of extensor and flexor cells in situ were only 1.01 times and 1.1 times larger, respectively, than at the point of incipient plasmolysis, whereas at full turgidity (cells in water) the corresponding factors were 1.8 and 1.5. It is suggested that the high elasticity of the longitudinal walls, the anisotropy of the cell walls, and the low water potential of the sections which is correlated with slightly stretched cell walls in situ, are favourable and effective for converting osmotic work in changes in length of the pulvinus cells, and thus for the up and down movement of the leaf.

Osmotica, dimethyl sulfoxide, parahydroxymercuribenzoate, and cyanide change the period of the circadian clock in the pulvini of Phaseolus coccineus L

Several substances change the period length of the circadian oscillator in the laminar pulvinus of Phaseolus coccineus L. if offered continuously via the transpiration stream to the isolated leaves in continuous light and constant temperature. The osmotica mannitol and PEG 6000 (polyethylene glycol) lengthen the circadian period from 27.9 hr (control) to about 29.5 hr. The dose-response of both substances reveal saturation curves. DMSO (dimethyl sulfoxide) lengthens the circadian period much more strongly than the osmotica. The dose-response curve reflects a curvilinear dependence of the period length from the DMSO concentration. PHMB (p-hydroxymercuribenzoate) and NaCN shorten the circadian period. The dose-response curve for PHMB is a saturation curve; saturation is reached at 80 microM PHMB with a period length of about 25.7 hr. We conclude from these results that the period of the circadian oscillator in the Phaseolus pulvinus is not directly homeostatically regulated but influenced by the intracellular milieu which can be changed by affecting membrane permeability, active transport systems or other regulatory systems within the cells.

Structure of the murine Ia-associated invariant (Ii) chain as deduced from a cDNA clone

The invariant (Ii) chain is a membrane-spanning glycoprotein found intracellularly associated with class II major histocompatibility complex (MHC) molecules. Using hybrid-selected translation and the Ii-specific monoclonal antibody In-1, we have isolated a cDNA clone (pIi-5) coding for most of the Ii chain. Sequence analysis of this clone reveals an open reading frame encoding 169 amino acid residues. The protein is rich in methionine and contains two potential N-glycosylation sites. No stretch of uncharged amino acid residues, characteristic for a membrane-spanning segment, is found close to the COOH-terminal end. There is one, however, close to the NH2-terminal end. As it is know that approximately 20 amino acid residues of Ii chain are exposed on the cytoplasmic side, we conclude that the Ii chain spans the membrane exposing the NH2 terminus on the cytoplasmic side and the COOH terminus on the luminal side.