Identical 3′ non-coding sequences in five mouse Ig κ chain mRNAs favour a unique C_κ gene

Abstract

The constant regions of immunoglobulin κ chains are encoded by a very small but unknown number of genes. The kinetics of nucleic acid reassociation is consistent with up to four C_κ genes per haploid mouse genome^1–4, while saturation hybridisation suggests that there are two C_κ genes⁵. The amino acid sequences proposed for seven κ C regions do differ slightly^6–10, but at least some of these differences result from sequencing errors¹¹. Determining the nucleotide sequence of the C region of κ mRNAs¹¹ and the adjacent 3′ non-coding region should more readily reveal any slight differences reflecting a multiplicity of C_κ genes. Examination of the 3′ non-coding region may be particularly useful, as this region is not constrained to encode the largely invariant C_κ amino acid sequence. Sequences reported for the 3′ terminal regions of two κ mRNAs, MOPC 21 (refs 11, 12) and MOPC 41A (ref. 13), were identical for the first 59 residues, but the subsequent, tentative, 45 residues proposed for MOPC 41A mRNA differed from the MOPC 21 sequence. It was unclear, however, whether these differences resulted from methodological problems, or genuine sequence divergence. We have therefore re-examined the MOPC 41A 3′ terminal sequence in a direct comparison with that of MOPC 21 and also determined 3′ terminal sequences of about 100 residues in three other κ mRNAs. We confirm here the sequence presented by Hamlyn et al.¹¹ for the MOPC 21 mRNA and show that not only the MOPC 41A mRNA, but also the three other κ mRNAs have the same sequence.