Recombination signal sequences

Recombination signal sequences are conserved sequences of noncoding DNA that are recognized by the RAG1/RAG2 enzyme complex during V(D)J recombination in immature B cells and T cells.^[1] Recombination signal sequences guide the enzyme complex to the V, D, and J gene segments that will undergo recombination during the formation of the heavy and light-chain variable regions in T-cell receptors and immunoglobulin molecules.^[1]

Structure

RSSs are made up of highly conserved heptamer sequences (7 base pairs), spacer sequences, and conserved nonamer sequences (9 base pairs) that are adjacent to the V, D and J sequences in the heavy-chain region of DNA and the V and J sequences in the light-chain DNA region.^[1][2] Spacer sequences are located between heptamer and nonamer sequences and exhibit base pair variety but are always either 12 base pairs or 23 base pairs long.^[3] Heptamer sequences are usually CACAGTG and nonamers are usually ACAAAAACC. The nucleotides in bold are more highly conserved.^[3] The RAG1/RAG2 enzyme complex follows the 12-23 rule when joining V, D, and J segments, pairing 12-bp spacer RSSs to 23-bp spacer RSSs.^[1][2] This prevents two different genes coding for the same region from recombining (ex. V-V recombination).^[1] RSSs are located between V, D, and J segments of the germ-line DNA of maturing B and T lymphocytes and are permanently spliced out of the final Ig mRNA product after V(D)J recombination is complete.^[1]

Function

The RAG1/RAG2 enzyme complex recognizes the heptamer sequences flanking the V and J coding regions and nicks their 5' end, releasing the intervening DNA between the V and J coding regions.

The RAG1/RAG2 enzyme complex recognizes the heptamer sequences flanking the V and J coding regions and nicks their 5' end, releasing the intervening DNA between the V and J coding regions.^[1] In the heavy-chain coding region of DNA, the RAG1/RAG2 enzyme complex recognizes the RSSs flanking the D and J segments and brings them together, forming a loop containing intervening DNA.^[1][4] The RAG1/RAG2 complex then introduces a nick at the 5' end of the RSS heptamers adjacent to the coding regions on both the D and J segments, permanently removing the loop of intervening DNA and creating a double-stranded break that is repaired by VDJ recombinase enzymes.^[1][4] This process is repeated for the joining of V to DJ.^[1] In light-chain rearrangement, only V and J segments are brought together.^[1]

Related Diseases & Disorders

cRSS

Cryptic RSSs are gene sequences that resemble authentic RSSs and are occasionally mistaken for them by the RAG1/RAG2 enzyme complex.^[3] Recombining an RSS with a cRSS can lead to chromosome translocations, which can lead to cancer.^[3]

Omenn's Syndrome

Some infants born with autosomal recessive SCIDS lack a functional copies of the genes that code for the RAG1/RAG2 enzyme complex because of missense mutations.^[5][6] These infants will produce a non-functional RAG1/RAG2 enzyme complex that cannot recognize RSSs and therefore cannot initiate V(D)J recombination effectively.^[5][6] This disorder is characterized by a lack of functioning B and T cells.^[1][5]