During a humoral immune response, immunoglobulin (Ig) class switching changes the effector function of an antibody, without altering its specificity for antigen, by replacing the heavy chain constant region (CH) sequences of the u gene with those of a gamma, epsilon or alpha gene. This is accomplished via recombination of DNA sequences (S regions) 5' of Cu and of the """"""""downstream"""""""" CH genes. According to the current model, regulation of class switch recombination (CSR) in response to specific B cell stimuli is achieved by modulation of the transcriptional activity of the target CH genes to generate so-called """"""""germline"""""""" (or """"""""switch"""""""") transcripts. The u locus plays a particularly important role in class switching, since Su is the obligate donor of every CSR event. Yet, the present models of CSR regulation do not satisfactorily explain all the features of u CSR activity, since the u locus switch transcripts are expressed throughout B cell development in the absence of detectable CSR activity. Moreover, deletions of the known regulatory and structural elements of the u switch transcripts have only a partial effect on u CSR. In a series of experiments stemming from these observations, we have recently identified novel u locus transcription units that may be involved in CSR regulation. Taking advantage of this new information, we propose here a series of experiments aimed at elucidating the regulatory mechanisms that affect CSR at the u locus. Specifically, we will characterize the elements that regulate expression of the new transcripts, and assess their function by targeted mutagenesis. To test whether generation and splicing of u switch transcripts is essential for CSR, as recently suggested for other CH genes, we are going to replace the entire u switch transcript region with exogenous DNA sequences. In transgenic experiments, we will assay for the role of cis- and trans-acting regulatory mechanisms in controlling Su recombination. Finally, by replacing the endogenous Su with different types of sequences (repetitive and non-repetitive, S-related or not) we will establish the molecular requirements for the Su region sequences as efficient CSR substrates.
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