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 Cκ 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 Vκ-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 Vκ-Cκ gene. This result, together with the fact that unrearranged Vκ genes are transcriptionally silent, suggests that structural features of both the Vκ and Cκ loci contribute to the overall transcriptional efficiency of a rearranged Vκ-Cκ 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.
Bibliographical noteFunding Information:
This research was supported by grants from the National Science Foundation and the U.S. Public Health Service: by fellowships from the Damon Runyan-Walter Winchell Fund to B. Van Ness and from the Cancer Research Institute to E. Mather; and by an appropriation from the Commonwealth of Pennsylvania.