The chicken avidin gene family : organization, evolution and frequent recombination

Abstract

Avidin is a biotin-binding protein found in chicken eggs. The gene encoding avidin (AVD) belongs to a family of multiple homologous genes, five of which (avidin-related genes 1-5 or AVRs 1-5) were previously cloned and sequenced. In this study, the chromosomal organization of the AVD and AVR genes was characterized. The gene family was localized on Zq21. Different sets of AVR genes were found upon screening genomic libraries, and two novel A VR genes (AVR6 and AVR7) were cloned and sequenced. The order and relative orientations of the AVR and AVD genes were mapped. A detailed evolutionary analysis of the gene sequences revealed that gene conversion plays an important role in the evolution of the gene family. Interestingly, the conversion showed a directional bias: AVD was able to convert the AVRs, but not vice versa. Recombinant AVR proteins were produced and preliminary analysis of their functional and structural properties were performed. Differences were found in some biochemical and structural characteristics of the AVRs as compared to avidin. The AVR proteins were shown to bind biotin, with AVR1 and AVR2 exhibiting partially reversible binding. The number of AVD and AVR genes was shown to differ between individuals, and also between cells within individuals. The molecular mechanism underlying the copy-number fluctuation seemed to be unequal crossing-over and/ or unequal sister chromatid exchange. The frequent occurrence of both gene conversion and unequal sequence exchange supports the model according to which conversion and crossing-over are coupled processes. The high frequency of recombination observed suggests that copy-number !ability may be more common among gene families than previously thought. Finally, the avidin gene family provides an excellent model for studying the molecular mechanisms of recombination events.