Cμ-containing transcripts initiate heterogeneously within the IgH enhancer region and contain a novel 5′-nontranslatable exon

Abstract

Transcriptional competence of the immunoglobulin heavy-chain locus (IgH) is established at an early stage of lymphoid cell development, leading to the appearance of RNA components, previously called C_μ RNA¹ or sterile-_μ RNA², which contain constant-region sequences but lack variable-region sequences. These components are of two types: those which initiate in the D region of alleles that have undergone DJ_H (diversity–joining region) rearrangement (D_μ transcripts) and those which initiate within the J_H–C_μ intron (hereafter termed I_μ transcripts)^3,4. In pre-B and early B cells, D_μ and l_μ transcripts are nearly as abundant as the messenger RNA encoding μ heavy chain^2,3,5. The D_μ transcripts are spliced into RNAs containing D, J_H and C_μ sequences, and in some, but not all, cases these RNAs are translated into D_μ proteins⁴. To establish whether the I_μ transcripts have any translational potential and to elucidate the structure of their promoter region, we have determined their transcription initiation sites and their mode of splicing. As reported here, by using sequence analysis of cloned I_μ complementary DNAs, primer extension and S₁, nuclease mapping, we have found that these transcripts have remarkable 5′ heterogeneity: ther e are more than five distinct start sites spanning a region of 44 nucleotides that is located downstream of an octanucleotide found in all variable-region promoters. Such imprecise initiation may result from the lack of a well-defined T A T AA motif and the unusual proximity of the octanucleotide to the enhancer region. Approximately 700 nucleotides downstream from these initiation sites, a cryptic splice site is used to create a nontranslatable exon (‘nontron’) which is joined to the C_μ1 domain. The properties of the nontron may be important for the mechanism of allelic exclusion.