Human J chain gene. Structure and expression in B lymphoid cells.

Abstract

As part of an ongoing investigation of the regulation of gene expression in B cell development, a genomic DNA clone encoding the human J chain protein was obtained. The nucleotide sequence of exons encoding the mature protein defines a 137 amino acid primary sequence similar to that previously determined at the protein level. Probes from the gene were used to analyze J chain expression in human cell lines corrsponding to pre-B and B lymphocytes. J chain RNA was detected in 2 of 6 human pre-B cell lines and in 8 of 10 B cell lines expressing various Ig isotypes. The expression of the J chain gene is, thus, not tightly linked to IgM or IgA secretion. These data do not, however, support the recent suggestion that synthesis of J chain precedes that of .mu. chain in B lymphocyte differentiation. Because of the presence of 9 candidate polyadenylation signals (AATAAA or AATTAAA) downstream of the C-terminal coding block of the J chain gene, the 3'' end of the gene could not be determined from sequence data alone. To define the 3'' end, J chain RNA from a human B lymphocyte line was used to protect an end-labeled DNA fragment from S1 nuclease digestion. The sequence 40 basepairs 5'' of the functional polyadenylation site identified by these S1 experiments is homologus in the same region of a previously reported mouse J chain complementary DNA clone.