Regulatory elements necessary for termination of transcription within the Ig heavy chain gene locus

Structural and functional properties of membrane and secreted IgD

More than 35 years ago, study of an unknown immunoglobulin (Ig) in the serum from a myeloma patient led to the discovery of IgD. Subsequently, the finding that it also exists as a membrane-bound Ig stimulated a large number of studies during the 70s. Then, the interest on IgD shrank, largely because of the lack of known function of secretory IgD (secIgD) and of a stagnating knowledge of the functions of surface IgD. In the recent years, very significant advances followed the tremendous accumulation of data on the physiology of the B cell receptor, of which IgD is the major component, on the role of secIgD in normal and diseased individuals. This review, which is focused on human IgD but integrates data in the mouse and other species when needed, summarizes present data on the structure, synthesis and functions of both membrane and secIgD, IgD receptors and the involvement of IgD in various diseases, especially the hyperIgD syndrome.

Differential regulation of transcription termination occurring at two different sites on the micro-delta gene complex

The progression of polymerases across the micro-delta Ig heavy chain gene complex is characterized by two termination events occurring at different sites on the transcription unit and at different times during B cell differentiation. We have utilized two mouse strains to analyze the regulatory determinants for these events in primary B cells. In the transgenic pmicro.microdeltaRatt strain a 1160 bp intervening DNA segment (the att site) has been inverted. This mutation results in the abrogation of transcription termination that occurs in early B cells. Using a novel method that takes advantage of an internal ribosome entry site we have further restricted the size of the segment that is needed for inducing transcription termination in transfectants. This 200 bp termination-inducing sequence operates in tumor equivalents of early but not mature B cells and the activity is correlated with differential binding of nuclear proteins. To explore the regulatory basis for the change in site of transcription termination upon B cell activation we have examined the microS-/- deletion mutant strain in which the microS poly(A) site has been eliminated. The results suggest that polyadenylation at the microS site plays a dominant but not exclusive role in regulating transcription termination in activated B cells.

Effect of upstream RNA processing on selection of mu S versus mu M poly(A) sites

All of the regulatory factors responsible for augmenting microseconds mRNA levels preceding the dramatic increase in secretory IgM production upon B cell activation has not been totally elucidated. Whereas previous experiments have centered on the region of the gene specifying the choice between splicing to mu M exons versus selection of the mu S poly(A) site, we have found that upstream sequences within the Cmu gene, specifically the Cmu 4 acceptor splice site together with intronic sequences between the Cmu 3++ and Cmu 4 exons, play an important role in dictating the precision or the extent of splicing to the mu M exons even under conditions in which functional polyadenylation factors should be in excess. Therefore, splicing of upstream exons can affect remotely located downstream exons. These findings suggest that regulation of differential mu S/mu M mRNA expression may involve general processing enzymes that recognize specific cis -regulatory sequences residing within the body of the mu gene and account for the unique ability of activated B cells to secrete copious amounts of IgM.

Poly(A)-driven and poly(A)-assisted termination: two different modes of poly(A)-dependent transcription termination

We mapped the elements that mediate termination of transcription downstream of the chicken betaH- and betaA-globin gene poly(A) sites. We found no unique element and no segment of 3'-flanking DNA to be significantly more effective than any other. When we replaced the native 3'-flanking DNA with bacterial DNA, it too supported transcription termination. Termination in the bacterial DNA depended on a functional poly(A) signal, which apparently compelled termination to occur in the downstream DNA with little regard for its sequence. We also studied premature termination by poorly processive polymerases close to the promoter. The rate of premature termination varied for different DNA sequences. However, the efficiencies of poly(A)-driven termination and promoter-proximal premature termination varied similarly on different DNAs, suggesting that poly(A)-driven termination functions by returning the transcription complex to a form which resembles a prior state of low processivity. The poly(A)-driven termination described here differs dramatically from the poly(A)-assisted termination previously described for the simian virus 40 (SV40) early transcription unit. In the SV40 early transcription unit, essentially no termination occurs downstream of the poly(A) site unless a special termination element is present. The difference between the betaH-globin and SV40 modes of termination is governed by sequences in the upstream DNA. For maximum efficiency, the betaH-globin poly(A) signal required the assistance of upstream enhancing sequences. Moreover, the SV40 early poly(A) signal also drove termination in betaH-globin style when it was placed in a betaH-globin sequence context. These studies were facilitated by a rapid, improved method of run-on transcription analysis, based on the use of a vector containing two G-free cassettes.

Alternative poly(A) site selection in complex transcription units: means to an end?

Many genes have been described and characterized which result in alternative polyadenylation site use at the 3'-end of their mRNAs based on the cellular environment. In this survey and summary article 95 genes are discussed in which alternative polyadenylation is a consequence of tandem arrays of poly(A) signals within a single 3'-untranslated region. An additional 31 genes are described in which polyadenylation at a promoter-proximal site competes with a splicing reaction to influence expression of multiple mRNAs. Some have a composite internal/terminal exon which can be differentially processed. Others contain alternative 3'-terminal exons, the first of which can be skipped in some cells. In some cases the mRNAs formed from these three classes of genes are differentially processed from the primary transcript during the cell cycle or in a tissue-specific or developmentally specific pattern. Immunoglobulin heavy chain genes have composite exons; regulated production of two different Ig mRNAs has been shown to involve B cell stage-specific changes in trans -acting factors involved in formation of the active polyadenylation complex. Changes in the activity of some of these same factors occur during viral infection and take-over of the cellular machinery, suggesting the potential applicability of at least some aspects of the Ig model. The differential expression of a number of genes that undergo alternative poly(A) site choice or polyadenylation/splicing competition could be regulated at the level of amounts and activities of either generic or tissue-specific polyadenylation factors and/or splicing factors.

Dependence of enhancer-mediated transcription of the immunoglobulin mu gene on nuclear matrix attachment regions

Transcription of the immunoglobulin mu heavy chain locus is regulated by an intronic enhancer that is flanked on both sides by nuclear matrix attachment regions (MARs). These MARs have now been shown to be essential for transcription of a rearranged mu gene in transgenic B lymphocytes, but they were not required in stably transfected tissue culture cells. Normal rates of transcriptional initiation at a variable region promoter and the formation of an extended deoxyribonuclease I (DNase I)--sensitive chromatin domain were dependent on MARs, although DNase I hypersensitivity at the enhancer was detected in the absence of MARs. Thus, transcriptional activation of the mu gene during normal lymphoid development requires a synergistic collaboration between the enhancer and flanking MARs.