During B-cell differentiation enhancer activity and transcription rate of immunoglobulin heavy chain genes are high before mRNA accumulation

During differentiation of B lymphocytes, a low level of immunoglobulin heavy chain gene transcripts is apparent at the pre-B-cell stage, and a dramatic increase in immunoglobulin mRNA level is seen after stimulated B cells have matured into immunoglobulin-secreting plasma cells. We have measured the transcription rate of endogenous heavy chain genes using cell lines representative of various stages of murine B-lymphocyte differentiation. We observe a good correlation between RNA polymerase density, as determined by nuclear run-on transcription experiments, and the activity of the heavy chain gene enhancer, as assayed by transfection experiments. Both enhancer activity and heavy chain gene transcription are very high in pre-B-cell lines. Thus we conclude that the increased accumulation of immunoglobulin heavy chain mRNA in immunoglobulin-secreting plasma cells is regulated mainly by posttranscriptional processes.