CD40-CD40L independent Ig gene hypermutation suggests a second B cell diversification pathway in humans

Somatically mutated IgM(+)-only and IgM(+)IgD(+)CD27(+) B lymphocytes comprise approximately 25% of the human peripheral B cell pool. These cells phenotypically resemble class-switched B cells and have therefore been classified as postgerminal center memory B cells. X-linked hyper IgM patients have a genetic defect characterized by a mutation of the CD40L gene. These patients, who do not express a functional CD40 ligand, cannot switch Ig isotypes and do not form germinal centers and memory B cells. We report here that an IgM(+)IgD(+)CD27(+) B cell subset with somatically mutated Ig receptors is generated in these patients, implying that these cells expand and diversify their Ig receptors in the absence of classical cognate T-B collaboration. The presence of this sole subset in the absence of IgM(+)-only and switched CD27(+) memory B cells suggests that it belongs to a separate diversification pathway.

Transcription, beta-like DNA polymerases and hypermutation

This paper discusses two aspects of immunoglobulin (Ig) gene hypermutation. In the first approach, a transcription termination signal is introduced in an Ig light chain transgene acting as a mutation substrate, and transgenic lines are generated with control and mutant transgenes integrated in tandem. Analysis of transcription levels and mutation frequencies between mutant and control transgenes clearly dissociates transcription elongation and mutation, and therefore argues against models whereby specific pausing of the RNA polymerase during V gene transcription would trigger an error-prone repair process. The second part reports the identification of two novel beta-like DNA polymerases named Pol lambda and Pol mu, one of which (Pol mu) represents a good candidate for the Ig mutase due to its higher lymphoid expression and its similarity with the lymphoid enzyme terminal deoxynucleotidyl transferase. Peculiar features of the expression of this gene, including an unusual splicing variability and a splicing inhibition in response to DNA-damaging agents, are discussed.

Two novel human and mouse DNA polymerases of the polX family

We describe here two novel mouse and human DNA polymerases: one (pol lambda) has homology with DNA polymerase beta while the other one (pol mu) is closer to terminal deoxynucleotidyltransferase. However both have DNA polymerase activity in vitro and share similar structural organization, including a BRCT domain, helix-loop-helix DNA-binding motifs and polymerase X domain. mRNA expression of pol lambda is highest in testis and fetal liver, while expression of pol mu is more lymphoid, with highest expression both in thymus and tonsillar B cells. An unusually large number of splice variants is observed for the pol mu gene, most of which affect the polymerase domain. Expression of mRNA of both polymerases is down-regulated upon treatment by DNA damaging agents (UV light, gamma-rays or H(2)O(2)). This suggests that their biological function may differ from DNA translesion synthesis, for which several DNA polymerase activities have been recently described. Possible functions are discussed.

A targeted deletion of a region upstream from the Jkappa cluster impairs kappa chain rearrangement in cis in mice and in the 103/bcl2 cell line

We have shown previously that a mutation of the KI-KII site immediately 5' to J(kappa)1 on the mouse immunoglobulin light chain kappa locus reduces the rearrangement level in cis, although it does not affect transcription. Here we deleted by homologous recombination in mouse embryonic stem cells a 4-kb DNA fragment, located immediately upstream of the KI-KII element, which contains the promoter of the long germline transcript. Analysis of gene-targeted heterozygous mouse splenic B cells showed a strong decrease in rearrangement for the allele bearing the deletion. When both the KI-KII mutation and the 4-kb deletion were present on the same allele, the overall reduction in rearrangement was stronger than with the 4-kb deletion alone underlying the role of these two elements in the regulation of rearrangement. The same deletion was performed by homologous recombination on one allele of the rearrangement-inducible mouse 103/bcl2-hygro(R) pre-B cell line, and resulted in a similar reduction in the induction of rearrangement of the mutated allele. This result validates this cell line as an in vitro model for studying the incidence of gene-targeted modifications of the kappa locus on the regulation of rearrangement.

Defect in IgV gene somatic hypermutation in common variable immuno-deficiency syndrome

Common Variable Immuno-Deficiency (CVID) is the most common symptomatic primary antibody-deficiency syndrome, but the basic immunologic defects underlying this syndrome are not well defined. We report here that among eight patients studied (six CVID and two hypogammaglobulinemic patients with recurrent infections), there is in two CVID patients a dramatic reduction in Ig V gene somatic hypermutation with 40-75% of IgG transcripts totally devoid of mutations in the circulating memory B cell compartment. Functional assays of the T cell compartment point to an intrinsic B cell defect in the process of antibody affinity maturation in these two cases.

Negative regulation of Ig gene rearrangement by a 150-bp transcriptional silencer

We previously showed that the V-J intervening sequence of the chicken lambda immunoglobulin locus contains a strong silencer that acts both on transcription and rearrangement. We show here that the transcriptional silencer activity can be ascribed to a minimal 150-bp fragment. The rearrangement silencing activity was previously shown by the replacement of the V-J intervening sequence with a neutral DNA fragment that dramatically increased the rate of rearrangement of the transgene. Insertion of the minimal silencer in this neutral fragment is shown here to result in a marked decrease in rearrangement of the transgenic construct. Strikingly, deletion of 28 bp from the 150-bp fragment abolished most of the transcriptional silencing activity and had a similar effect on rearrangement. These results conclusively correlate the silencing activity on both rearrangement and transcription.

Probing immunoglobulin gene hypermutation with microsatellites suggests a nonreplicative short patch DNA synthesis process

As the rate of Ig gene hypermutation approximates the level of nucleotide discrimination of DNA polymerases (10(-3) to 10(-4)), a local inhibition of proofreading and mismatch repair during semiconservative replication could generate the mutations introduced by the process. To address this question, we have constructed transgenic mice that carry a hypermutation substrate containing a "polymerase slippage trap": an Ig gene with a mono or dinucleotide tract inserted in its V region. The low amount of slippage events as compared to the number of mutations, the absence of transient misalignment mutations at the border of the repeats, and the dissociation between the amount of frameshifts and mutations when the transgene is put on mismatch repair-deficient genetic backgrounds, suggest that Ig gene hypermutation occurs by an error-prone short patch DNA synthesis taking place outside global DNA replication.

Mismatch repair deficiency interferes with the accumulation of mutations in chronically stimulated B cells and not with the hypermutation process

Primary responses to the hapten phenyloxazolone and chronic responses to environmental antigens occurring in Peyer's patches were analyzed in two different mismatch repair-deficient backgrounds. Paradoxically, whereas primary responses were found normal in MSH2- and only slightly diminished in PMS2-deficient mice, mutations in Peyer's patch B cells from both k.o. animals were reduced three times, the subset of Peyer's patch B cells with highly mutated sequences being specifically missing in the mismatch repair-deficient context. Strikingly, germinal center B cells from Peyer's patches of k.o. animals showed microsatellite instability at an unprecedented level. We thus propose that the amount of DNA damages generated prevents these cells from recycling in germinal centers and that mismatch repair deficiency is only the indirect cause of the lower mutation incidence observed.

Generation of diversity in mammalian gut-associated lymphoid tissues: restricted V gene usage does not preclude complex V gene organization

The sheep genome contains 60 to 90 V lambda genes distributed in least 6 different families, whereas 37 V lambda genes and 10 families exist in humans. Comparison with human V lamda sequences indicates that sheep V lambda genes display less overall sequence divergence, but are closer to the genes most frequently used in the human peripheral repertoire. In both species, 2 to 3 genes contribute half of the expressed sequences. Therefore, similar large combinatorial potential and restriction of the expressed repertoire can exist in two species whose strategy of diversification differs widely (ongoing rearrangement throughout life in human bone marrow vs postrearrangement diversification during early development in sheep ileal Peyer's patches).

Generation of diversity in mammalian gut-associated lymphoid tissues: restricted V gene usage does not preclude complex V gene organization

The sheep genome contains 60 to 90 V lambda genes distributed in least 6 different families, whereas 37 V lambda genes and 10 families exist in humans. Comparison with human V lamda sequences indicates that sheep V lambda genes display less overall sequence divergence, but are closer to the genes most frequently used in the human peripheral repertoire. In both species, 2 to 3 genes contribute half of the expressed sequences. Therefore, similar large combinatorial potential and restriction of the expressed repertoire can exist in two species whose strategy of diversification differs widely (ongoing rearrangement throughout life in human bone marrow vs postrearrangement diversification during early development in sheep ileal Peyer's patches).

Rearrangement-enhancing element upstream of the mouse immunoglobulin kappa chain J cluster

Transcriptional regulatory elements have been shown to be necessary but not sufficient for the developmental regulation of immunoglobulin gene rearrangement in mouse precursor B cells. In the chicken lambda light chain locus, additional elements in the V-J intervening sequence are involved in negative and positive regulation of rearrangement. Here, mutation of the mouse homolog of a chicken element, located in the V(K)-J(K) intervening sequence upstream of the J(K) cluster, was shown to significantly decrease rearrangement. This cis-acting recombination-enhancing element affects the rearrangement process without being involved in regulating transcription.

Rearrangement/hypermutation/gene conversion: when, where and why?

Diversification strategies for immunoglobulins vary widely in different species. Here, Jean-Claude Weill and Claude-Agnès Reynaud argue that V(D)J recombination arose as a means for achieving allelic exclusion rather than diversity, and postulate that the choice of a diversification strategy is selected along with a specific site of B-cell differentiation. They propose that somatic mutation and gene conversion represent analogous molecular strategies occurring in specific chromatin accessibility contexts.

Hypermutation generating the sheep immunoglobulin repertoire is an antigen-independent process

Somatic hypermutation of light chain V genes during development of B cells in sheep ileal Peyer's patches was studied in three experimental conditions: in sterile fragments of the ileum surgically isolated from the gut during fetal life, in germ-free sheep, and in animals thymectomized during early fetal life. The somatic mutation pattern was found identical to control tissues in all three experiments. The same age-dependent amount of mutations, a higher than theoretical R/S ratio in complementarity-determining regions (CDRs), and a similar clustering of mutations in CDRs were observed. The mechanism, as estimated from the silent mutation pattern, appears to target mutations to CDRs; moreover, the major V lambda genes have a specific codon usage with a high purine content at the first two bases of the codons and a low content at the third position, which, together with a specific targeting of mutations to purines, favors replacement mutations in CDRs.

Rearrangement of the chicken lambda light chain locus: a silencer/antisilencer regulation

Chicken immunoglobulin lambda light chain rearrangement is regulated by different controlling elements. One negative control element, acting as a strong transcriptional silencer, is located in the V-J intervening sequence which is excised during the rearrangement process. Positive control elements include the V lambda promoter, the enhancer of transcription (3' of C lambda), and one (or two) putative antisilencer element(s) located on each side of the silencer. It is proposed that these antisilencer elements counteract transiently the silencer thus allowing rearrangement of one allele in chicken B cell progenitors. The implications of this regulation for mouse B cell development are discussed.

Promoter, enhancer and silencer elements regulate rearrangement of an immunoglobulin transgene

The chicken Ig lambda light chain locus is composed of a single V gene closely linked (1.8 kb) to a single J-C unit in its natural configuration. In mice transgenic for this locus, the transgene becomes rearranged in B cells and to a much lesser extent in T cells. Modifications were introduced in the transgene in order to characterize elements which target the recombinase to the Ig loci. In the absence of either the promoter or the enhancer located 3' of C lambda, rearrangement of the transgene is reduced 20- to 100-fold. Moreover, rearrangement is increased 5-fold when the DNA segment between V lambda and J lambda ('Uo segment'), which is deleted during the joining process, is replaced by a neutral DNA segment of equal length. The Uo segment behaved as a strong transcriptional silencer when tested in a CAT assay in vitro. Control transgenic mice harbouring only the two 3 bp mutations that introduced restriction sites at both ends of the Uo segment to allow for its replacement were also analysed. Rearrangement was reduced 10- to 100-fold in B cells from such transgenic lines. A model is proposed whereby the sites of these two mutations would function by counteracting transiently the repressing effect of the silencer, thus giving access of the chicken light chain locus to the recombinase.

Emergence of committed B lymphoid progenitors in the developing chicken embryo

The formation of B lymphoid restricted progenitors was followed during chicken embryonic development by monitoring the appearance of the various Ig gene rearrangements (DJH, VHDJH, V lambda J lambda), as a sensitivity that allows the detection of a single rearranged cell. By quantifying the DJH committed progenitor populations, we describe their evolution in different compartments at different developmental stages. The yolk sac is the first site where DJH-positive cells are observed (at days 5-6 of development); via the general circulation, they then seed the various organs while undergoing VHDJH and V lambda J lambda rearrangements, which occur simultaneously but lag behind DJH by one to several days. These progenitor populations decline with time in most lymphoid sites and only expand in the bursa. RAG-1 expression is observed in the bursa in the absence of ongoing rearrangement activity and thus appears to be an improper marker of rearrangement in the chicken. Commitment to the B cell lineage seems to result from an intrinsic cell program, but the survival and expansion of the committed B progenitors require the specific microenvironment of the bursa.

Early B-cell development in chickens, sheep and rabbits

Studies of the immune system of various species have revealed that antibody repertoire can be generated in many different ways. This review underlines some general principles for comparing the different processes which represent the basic framework of these systems.

The chicken D locus and its contribution to the immunoglobulin heavy chain repertoire

Sixteen D elements were characterized from the chicken genome, 15 of which are extremely homologous. Early expression of this D repertoire was studied for both DJ and VDJ alleles. No N diversification occurs at either DJ or VD junctions. Only P additions were observed, the length of which does not appear restricted to a dinucleotide. A selection for the almost exclusive usage of the first reading frame of the D elements takes place during B cell expansion in the bursa, in parallel with the selection of productive rearrangements. All three reading frames were observed for the DJ allele at each developmental stage, although some bias for the first reading frame occurs already at the junctional stage. The high incidence of D-D junctions observed (25% among DJ sequences) might represent the major functional contribution of this multigene cluster in a system in which diversity will be generated later on by successive superimposed gene conversions. Other possible functions are discussed. The onset of D diversification through gene conversion between day 15 and day 18 of embryonic development is further documented.

Somatic generation of diversity in a mammalian primary lymphoid organ: the sheep ileal Peyer's patches

Ileal Peyer's patches (IPPs) in the sheep are composed of tightly packed follicles in which surface IgM-positive B cells proliferate and can be exported to the periphery. We report that the light chain rearrangement pattern in a single IPP follicle is much more restricted than in the entire tissue, which indicates that, as in the chicken bursa, ongoing rearrangement does not take place in this organ. Moreover, we show that B cells extensively diversify their antigen receptor while proliferating in IPP follicles. Sequencing of part of the V lambda locus indicates that this diversification is not achieved by gene conversion, but rather by untemplated somatic mutation and intense selective pressure. These results strongly imply that sheep IPPs behave as a bursa-equivalent, primary lymphoid organ of diversification and that somatic point hypermutation, which is known to proceed during secondary immune responses, can also generate an antibody repertoire.

Light chain gene conversion continues at high rate in an ALV-induced cell line

We have analyzed immunoglobulin light chain sequences from avian leukosis virus (ALV) induced bursal and metastatic tumors and from cell lines derived from these tumors. Sequence data presented demonstrate that ALV-induced tumors and one cell line (DT40) derived therefrom continue to diversify their light chain genes outside of the bursal environment. Diversification within these tumor cells seems to occur by gene conversion events comparable with those observed in bursal B cells. Sequence analysis of spontaneously arising surface immunoglobulin negative subclones of the DT40 cell line revealed frameshifts within the rearranged light chain genes which most likely resulted from non-functional recombination events. Superimposed gene conversion events can repair these frameshifts leading to re-expression of surface immunoglobulin.

Somatic hyperconversion diversifies the single Vh gene of the chicken with a high incidence in the D region

The chicken heavy chain locus contains a single JH segment and a unique functional VH gene (VH1) 15 kb upstream, with approximately 15 D elements in between. A cluster of pseudogenes (psi VH) spans 60-80 kb, starting 7 kb upstream from VH1, with an average density of one pseudogene per 0.85 kb and an almost systematic alternation of polarity. Diversification of the unique rearranged VH1 gene takes place during bursal ontogeny by the same hyperconversion mechanism that was described for the chicken light chain, with psi VH segments acting as donors. The hyperconversion mechanism also operates within the D region, as all pseudogenes analyzed are fused VD elements; this D region possesses distinct characteristics, allowing higher combinatorial possibilities in the gene conversion process. Allelic exclusion appears to be performed by restriction of a complete VDJ rearrangement to a single allele.

The chicken B cell compartment

A very unusual molecular mechanism is involved in generating the preimmune repertoire in the chicken bursa of Fabricius. A unique rearranged V gene is diversified through a program of segmental gene conversion with a pool of noncoding pseudogenes being used as donors. A specifically committed progenitor that originates in the embryonic bursa is responsible for long-term maintenance of the B cell population. Both these properties and the characteristics of the peripheral B cell compartment are discussed in terms of the evolution of the T and B immune systems.

Rearrangement of a chicken immunoglobulin gene occurs in the lymphoid lineage of transgenic mice

Immunoglobulin (Ig) and T-cell antigen receptor genes rearrange through identical heptamer-nonamer recognition sequences during entry of cells into the B or T lymphoid lineage. A similar enzymatic machinery may be used to perform these highly cell-specific events in these two types of lymphoid cells. We have investigated what the signal may be that triggers the rearrangement of one or other of the receptor genes in B or T cells. Mice from three transgenic lines carrying two, four or twenty copies of the unrearranged chicken lambda light-chain locus were analysed. In all three lines the chicken Ig transgene rearranges in B cells; in the line with 20 copies, a rearranged fragment can also be detected in thymus DNA. We conclude that the inserted chicken light-chain locus in its natural configuration contains target sequences that permit specific rearrangement in mouse lymphoid B cells, but that this precise differentiation step may be deregulated in thymic cells when the physiological level of relevant information is experimentally altered.

A hyperconversion mechanism generates the chicken light chain preimmune repertoire

The chicken immunoglobulin light chain repertoire has been shown to be entirely derived from a single V lambda 1-J rearranged combination. The complete coding information of the lambda locus was determined: it comprises 25 V-hybridizing elements, all of which are pseudogenes, clustered in both orientations within 19 kb of DNA, starting 2.4 kb upstream of the V lambda 1 gene. Sequences of somatically rearranged V lambda 1 genes from embryonic and posthatching bursal cells show that diversification of light chain sequences occurs during ontogeny by a segmental gene conversion mechanism which takes place at a frequency of 0.05-0.1 per cell generation between the pseudogene pool and the unique rearranged functional V gene.

Rearrangement of chicken immunoglobulin genes is not an ongoing process in the embryonic bursa of Fabricius

We report a molecular analysis of the chicken Ig loci in single bursal follicles from 3- to 7-week-old chickens. Each follicle contained between 10(5) and 3 X 10(5) cells. The Ig gene rearrangement patterns obtained were compared to the pattern observed with the corresponding total bursal DNA. The results obtained for the light chain locus imply that a very small number (two on average) of rearrangement events takes place in each follicle. For the heavy chain locus similar results were obtained, each follicle showing a more restricted pattern than the total bursa. These data favor a model in which each follicle is colonized by a very few prebursal stem cells that are committed to a particular Ig gene rearrangement at the very beginning of the development of the embryonic bursa. The role of the bursa as the organ in which such a committed stem cell population for the B-cell lineage arises is discussed.

A single rearrangement event generates most of the chicken immunoglobulin light chain diversity

The chicken immunoglobulin lambda locus contains a single C lambda gene with a unique J lambda element, 1.9 kb upstream. The same V lambda gene (V lambda 1) is rearranged in most cells of the Bursa of Fabricius. This V lambda 1 gene is located, in germ-line configuration, 1.7 kb upstream from J lambda and in the same transcriptional orientation. Eight to twelve variable genes of the same set are found adjacent to the V lambda 1 gene, indicating that V-gene amplification did occur. Three of these genes were sequenced and proved to be pseudogenes, one of them having an inverted polarity. Data suggesting extensive somatic diversification of the V lambda 1 sequence are reported, including the possible use of nonfunctional V elements in a somatic gene-conversion-like process.

Nucleotide sequence of the constant region of a chicken mu heavy chain immunoglobulin mRNA

We have recently reported the sequence of a chicken Ig lambda light chain cDNA clone, isolated from a spleen partial cDNA library (1). In this paper, we describe the characterization of a cDNA clone coding for the chicken constant (C) region of the secreted mu chain. This is the first report on the nucleotide and amino acid sequence of a chicken Ig heavy chain constant region. It contains the 3' untranslated region of the mu mRNA up to the poly(A) tail, and, in comparison with the mouse Cmu sequence, displays the overall domain size and organization of a secreted mu chain, i.e.: a characteristic COOH-terminal region, a Cmu4, a Cmu3, a Cmu2, and part of a Cmu1 domain. The sequence homology between these two species ranges from 45% for the Cmu4 to 18% for the Cmu2. Thus, the Cmu sequence appears much less conserved between chicken and mouse than their respective lambda light chain constant regions (1). These results, together with some distinctive features of the Cmu2 domain, may be of evolutionary relevance and will be further discussed.

Complete sequence of a chicken lambda light chain immunoglobulin derived from the nucleotide sequence of its mRNA

Recombinant cDNA plasmids have been constructed from chicken spleen poly(A)-containing RNA. Two clones have been selected and provide the sequence determination of a chicken lambda immunoglobulin light chain: they include the complete variable, constant, and 3' untranslated regions of the chicken lambda light chain mRNA and part of the leader sequence. Comparison of the chicken light chain constant region with both human and mouse lambda constant sequences indicates 61% homology at the amino acid level. Unexpectedly, the chicken variable sequence is 53-63% homologous to human variable sequences when it is compared to the various lambda subgroups and only 42% homologous to the mouse V lambda 1 sequence. The degree of homology between the variable regions of these three species does not easily correlate with their phylogenetic relationship.

Modification of the methylation pattern in the vicinity of the chicken globin genes in avian erythroblastosis virus transformed cells

Having previously found a reduced transcription of globin genes and an abortive processing of the already transcribed globin pre-mRNA in Avian Erythroblastosis Virus (AEV) transformed cells (1), we compared the genomic DNA of these cells with that of normal chicken erythroblasts, using 32-P-labelled cDNA probes specific for the beta, alpha A and alpha D globin sequences. Restriction endonuclease digestion, electrophoresis of digests in agarose gels, Southern blotting and hybridization were carried out. Our results show that the overall genome organization is not disturbed in the immediate neighbourhood of the adult globin genes; the observed restriction fragments are identical for both DNAs after EcoRI, HindIII, BamHI and XbaI digestion, using the beta, alpha A and alpha D globin cDNA probes. However, we observe specific modifications at some methylation sites in the beta, beta-like and alpha D regions: after HpaII or MspI digestion in the alpha D region and after HhaI digestion in the beta and beta-like region, heavier bands appear in the transformed cell DNA in addition to the ones observed in normal DNA. This implies that, at some specific sites, the transformed cell DNA is more methylated than the normal erythroblast DNA. The possible significance of this observation is discussed.

Restriction mapping of cDNA recombinants including the adult chicken and duck globin messenger sequences: a comparative study

A comparison of the organization of six avian adult globin messenger sequences is based on previously reported recombinant duck adult globin cDNA plasmids (Therwath et al., 1980) and the actual construction and characterization of pBR322 recombinant plasmids including the beta and the normal alpha A and alpha D chicken adult globin mRNA sequences. Identification of the cloned DNA was performed using hybridization-selection under conditions permitting complete purification in one step of the three globin mRNAs, and translation of the corresponding mRNA. Orientation of the globin insert in the vector was determined, taking into account the computer prediction of the restriction sites based on the known amino acid sequences of the three globin chains (Roizès and Pelaquier, 1980) and those actually observed, and by identification of restriction fragments using 3'-specific probes. Identification, orientation and restriction mapping of these cloned DNAs reveals extensive homologies in organisation of beta sequences between duck and chicken, as well as among the alpha sequences in every two possible combinations.

On pre-messenger RNA and transcriptions. A review

From the present review integrating old and new data emerge a few principles of gene expression in eukaryotes, and an infinite variety of possible mechanistic details generating the overal pattern. The few principles, most of which are not fundamentally new, may thus be summarized. 1) The eukaryotic genome is subdivided into transcriptional units: into transcriptons which are subject to individual activation controlled at DNA level. 2) Viral genomes may contain one or a few transcriptons, while cells of multicellular organisms contain from 3 x 10(3) in diptera up to an estimated 2 x 10(5) in birds and mammals. 3) Transcriptons may include one or several coding sequences. 4) Transcriptons vary considerably in size: in mammals and birds their size spectrum falls into the 2,000 to 20,000 bp range. 5) Units of coding information constituting one message (genes) and, possibly, units of regulative information are frequently broken up and stored within the transcripton in sub-genic blocks (of so far unknown significance) in general located at a certain distance from the 5' and 3' transcript terminals which are determined by the promotor and terminator signals. 6) The gene, in its specific definition as the functional unit underlying the phenotype, is in general constituted posttranscriptionally by the processing mechanisms from the mosaic of its genomic subunits in the transcripton; segments of coding, service and regulative sequences are recombined within themselves and with each other, polygenic transcripts separate into their unit messages. 7) Activated transcriptons produce pre-mRNA; these primary transcripts are colinear with the DNA of the transcriptional unit. 8) Primary pre-mRNA is processed into secondary pre-mRNA's by extragenic cleavage and intragenic ("splicing") processing, giving rise stepwise to functional mRNA. During this process chemical modifications as methylation, 5'-terminal capping and 3'-terminal polyadenylation take place. 9) Translation yields either potentially functional polypeptides or polycistronic polyproteins subject to further processing. 10) Processing is a regulated process; it involves many of the possible phases and mechanisms of post-transcriptional regulation (cf. 39, 40).