Germline transcription of the murine immunoglobulin gamma-1 locus: Characterization of the promoter elements in transgenic mice.

Abstract

During the murine immune response, mice can produce eight classes of antibody molecules. These classes are defined by the heavy chain constant regions which are encoded by genes found on chromosome 12. Naive B cells express antibodies of the IgM and IgD classes on their surfaces, but upon antigenic stimulation, the cells can change the class of antibody produced to one of the other six classes. This class switching is effected by a DNA recombination event between specialized DNA sequences (switch regions) found upstream of each of the constant region genes. After the recombination event, the gene encoding the antigen binding domain of the antibody is juxtaposed to the gene for a downstream heavy chain constant region. In part of this work, the insertion of an E.Tn element into the $\gamma$2a switch region of plasmacytoma P3.26Bu4 was characterized. The analysis suggested the involvement of switch recombination enzymes in this insertion of the retrotransposon. Prior to the recombination event, the constant region genes are transcribed in their germline configuration. This transcription initiates in a region upstream of the switch region and is induced by cytokines. The main part of this thesis investigated the cis-acting elements that regulate the germline transcription of the IgGl gene in a transgenic mouse system. We generated transgenic mice with promoter region fragments directing the transcription of reporter genes, and characterized their expression patterns. The promoter fragments included 3500, 2300, 1700, or 335 basepairs of sequences near the transcription start sites. Another construct contained 17 kilobases, beginning 2100 basepairs upstream of the start sites and extending downstream through the entire switch and constant regions. Our results indicated that the 17 kilobase construct contained all of the elements required for regulated transcription of the locus. All of the smaller constructs, including the 335 basepair promoter region construct, are transcribed almost exclusively in lymphoid cells; but they lack a negative element found in the 17 kilobase construct that prevents inappropriate expression.