Trans-acting nuclear protein responsible for induction of rearranged human immunoglobulin heavy chain gene

A complete human gamma 1 heavy chain gene (HIG1) was transferred into mouse cells by protoplast fusion. The HIG1 gene was strongly expressed in mouse myeloma cells but not in mouse fibroblasts (L cells). Nuclear extracts from myeloma cells were injected into L cell transformants containing one copy of the HIG1 gene; this triggered accurate transcription of the HIG1 gene in the transformants. The induction of HIG1 gene expression by a myeloma nuclear factor (or factors) appeared to depend on the enhancers in the heavy chain gene. Nuclear proteins prepared from cells of the B lineage could induce the transcription of HIG1 gene in the L cell transformants, while those from the cells of non-B-lineage could not. The present study shows that positive regulatory trans-acting factors are involved in the activation of the immunoglobulin heavy chain gene through its enhancers and are contained only in cells of the B lineage.