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.