Transcription of the unrearranged mouse C_κ locus: Sequence of the initiation region and comparison of activity with a rearranged V_κ-C_κ gene

In cells of the B-lymphocyte lineage, 8.4 kb transcripts are constitutively produced from unrearranged kappa constant region (κ⁰) loci. To help elucidate the molecular basis of this phenomenon, we have determined the nucleotide sequence surrounding the site of transcriptional initiation. The κ⁰ transcripts are initiated within a unique Eco RI fragment located about 8 kb upstream from the C_κ gene. The start site is about 36 nucleotides downstream from a Hogness consensus sequence (TGTAAAT) and nearly 200 nucleotides upstream from a sequence that is similar to those encoding the signal peptides of κ light chains. These features, which are usually found in the 5′ flanking regions of kappa variable region genes, suggest that the κ⁰ initiation sequence may be an evolutionary relic of some common ancestral 5′ element. In contrast, there is no discernible V_κ-encoding element in 780 nucleotides of sequence downstream from the initiation site. From pulse-chase-labeling experiments with a pre-B-cell hybridoma line and direct measurements of transcriptional activity in isolated nuclei, we have estimated that the rate of transcription of the κ⁰ locus is significantly lower than that of a rearranged V_κ-C_κ gene. This result, together with the fact that unrearranged V_κ genes are transcriptionally silent, suggests that structural features of both the V_κ and C_κ loci contribute to the overall transcriptional efficiency of a rearranged V_κ-C_κ gene. The 8.4 kb transcripts are not processed into any stable RNA products, despite the fact that they contain some apparently normal splice junctions; rather, they are degraded within the nucleus at about half the rate with which a κ mRNA precursor is processed. Conceivably, the transcriptional activity of the κ⁰ locus might be a prerequisite for its recombinatorial activity.