Question

1. How do chickens create such a large variety of lambda type light chains from a single functional V gene segment? Diagram and explain.

Answer 1

The chicken Ig locus is a typical example for the Ig somatic diversification mechanismknown as gene conversion which is found in rabbits, cows and pigs. A similar mechanism is exploited by the single (? like) L chain locus and the H chain loci. One functional V gene segment, one J_? segment and one C_? gene segment are present in the chicken ? locus. 25 V_? pseudogenes which are organized in either orientation present upstream of the functional V_?1 gene segment. In pseudogenes, 3 possibilities are present:

1. The coding segment is deleted in one or both ends
2. Proper RSSs are missing
3. Both 1 and 2.

This is asserted by the fact that only V_?1 gene segment recombines with the J_?C_? gene segment. However, sequences of arranged V_?J_?-C_? gene segments appear in considerable diversity. In a rearranged gene,a cluster of changes occurred in its sequences at one or more positions. A sequence identical to the new sequence can be always found in one of the pseduogenes. If the sequence are not found in a pseudogene, it always represent changes at the junctinon of the original sequence and the altered sequence. The unmodified V_?1 sequence is not epressed until the sequences from the pseudogenes (10 to 120bp) are inserted into the active v_?1 region by gene conversion. A successful gene conversion event probably occurs every 10 to 20 cell divisinos or to every rearranged V_?1 sequence which occur at the end of the immune maturation period.Thus it can produce 2.5*108 possible combinations.