8536d_ch05_105-136 8/22/02 2: 46 PM Page 111 mac46 mac46: 1256_deh: 8536d: Goldsby et al./Immunology 5e Organization and Expression of Immunoglobulin Genes CHAPTER 5 111 assembled by recombinational events at the DNa level. These Light-Chain DNA Undergoes vents and the parallel events involving T-receptor genes are V- Rearrangements the only known site-specific DNA rearrangements in verte brates. Variable-region gene rearrangements occur in an or- Expression of both k and A light chains requires rearrange dered sequence during B-cell maturation in the bone marrow. ment of the variable-region V and J gene segments In hu The heavy-chain variable-region genes rearrange first, then mans, any of the functional Va genes can combine with any the light-chain variable-region genes. At the end of this of the four functional Jx-Cx combinations In the mouse process, each B cell contains a single functional variable- things are slightly more complicated. DNA rearrangement region DNA sequence for its heavy chain and another for its can join the val gene segment with either the Jal or the J.3 gene segment, or the V,2 gene segment can be joined witl The process of variable-region gene rearrangement pro- the Ja2 gene segment. In human or mouse k light-chain duces mature, immunocompetent B cells; each such cell is DNA, any one of the Vk gene segments can be joined with committed to produce antibody with a binding site encoded any one of the functional J gene segments by the particular sequence of its rearranged V genes. As de- Rearranged K and A genes contain the following regions in scribed later in this chapter, rearrangements of the heavy- order from the 5 to 3 end: a short leader(L)exon, a non chain constant-region genes will generate further changes in coding sequence(intron), a joined V] gene segment, a second he immunoglobulin class(isotype)expressed by a B cell, but intron, and the constant region. Upstream from each leader hose changes will not affect the cells antigenic specificity. gene segment is a promoter sequence. The rearranged light- The steps in variable-region gene rearrangement occur in chain sequence is transcribed by RNA polymerase from the L an ordered sequence, but they are random events that result exon through the C segment to the stop signal, generating a in the random determination of B-cell specificity. The order, light-chain primary RNa transcript( Figure 5-4). The in mechanism, and consequences of these rearrangements are trons in the primary transcript are removed by RNA- described in this section processing enzymes, and the resulting light-chain messenge K-chain dna 5 HHHH v-J Jx Jx CK …LH adenylation RNA splicing mRNA L VJ CK Nascent polypeptide LVJ CK FIGURE 5.4 Kappa light-chain gene rearrangement and RNA pro- cessing events required to generate a k light-chain protein example, rearrangement joins V23 and J4assembled by recombinational events at the DNA level. These events and the parallel events involving T-receptor genes are the only known site-specific DNA rearrangements in verte￾brates. Variable-region gene rearrangements occur in an or￾dered sequence during B-cell maturation in the bone marrow. The heavy-chain variable-region genes rearrange first, then the light-chain variable-region genes. At the end of this process, each B cell contains a single functional variable￾region DNA sequence for its heavy chain and another for its light chain. The process of variable-region gene rearrangement pro￾duces mature, immunocompetent B cells; each such cell is committed to produce antibody with a binding site encoded by the particular sequence of its rearranged V genes. As de￾scribed later in this chapter, rearrangements of the heavy￾chain constant-region genes will generate further changes in the immunoglobulin class (isotype) expressed by a B cell, but those changes will not affect the cell’s antigenic specificity. The steps in variable-region gene rearrangement occur in an ordered sequence, but they are random events that result in the random determination of B-cell specificity. The order, mechanism, and consequences of these rearrangements are described in this section. Light-Chain DNA Undergoes V-J Rearrangements Expression of both and light chains requires rearrange￾ment of the variable-region V and J gene segments. In hu￾mans, any of the functional V genes can combine with any of the four functional J-C combinations. In the mouse, things are slightly more complicated. DNA rearrangement can join the V1 gene segment with either the J1 or the J3 gene segment, or the V2 gene segment can be joined with the J2 gene segment. In human or mouse light-chain DNA, any one of the V gene segments can be joined with any one of the functional J gene segments. Rearranged and genes contain the following regions in order from the 5 to 3 end: a short leader (L) exon, a non￾coding sequence (intron), a joined VJ gene segment, a second intron, and the constant region. Upstream from each leader gene segment is a promoter sequence. The rearranged light￾chain sequence is transcribed by RNA polymerase from the L exon through the C segment to the stop signal, generating a light-chain primary RNA transcript (Figure 5-4). The in￾trons in the primary transcript are removed by RNA￾processing enzymes, and the resulting light-chain messenger Organization and Expression of Immunoglobulin Genes CHAPTER 5 111 FIGURE 5-4 Kappa light-chain gene rearrangement and RNA pro￾cessing events required to generate a light-chain protein. In this example, rearrangement joins V23 and J4. Germ-line κ-chain DNA 5′ Vκ1 Vκ23 Vκn Jκ Cκ 3′ 3′ Vκ Jκ Vκ Jκ Jκ Jκ Cκ Rearranged κ-chain DNA V-J joining 3′ Cκ Transcription Primary RNA transcript mRNA VJ Cκ Nascent polypeptide VJ Cκ V J κ light chain Cκ Polyadenylation RNA splicing Translation Vκ Cκ (A)n Poly-A tail ψ L 5′ Vκ1 5′ L L L L L L L 8536d_ch05_105-136 8/22/02 2:46 PM Page 111 mac46 mac46:1256_des:8536d:Goldsby et al. / Immunology 5e: