Transcription of the unrearranged mouse C kappa locus: sequence of the initiation region and comparison of activity with a rearranged V kappa-C kappa gene.

In cells of the B-lymphocyte lineage, 8.4 kb transcripts are constitutively produced from unrearranged kappa constant region (kappa 0) loci. To help elucidate the molecular basis of this phenomenon, we have determined the nucleotide sequence surrounding the site of transcriptional initiation. The kappa 0 transcripts are initiated within a unique Eco RI fragment located about 8 kb upstream from the C kappa 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 kappa light chains. These features, which are usually found in the 5' flanking regions of kappa variable region genes, suggest that the kappa 0 initiation sequence may be an evolutionary relic of some common ancestral 5' element. In contrast, there is no discernible V kappa-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 kappa 0 locus is significantly lower than that of a rearranged V kappa-C kappa gene. This result, together with the fact that unrearranged V kappa genes are transcriptionally silent, suggests that structural features of both the V kappa and C kappa loci contribute to the overall transcriptional efficiency of a rearranged V kappa-C kappa 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 kappa mRNA precursor is processed. Conceivably, the transcriptional activity of the kappa 0 locus might be a prerequisite for its recombinatorial activity.