A MODEL FOR THE EVOLUTION OF POLYUBIQUITIN GENES FROM THE STUDY OF ARABIDOPSIS-THALIANA ECOTYPES

Polyubiquitin genes encode the highly conserved 76-amino acid protein ubiquitin that is covalently attached to substrate proteins targeting most for degradation. Polyubiquitin genes are characterized by the pre sence of tandem repeats of the 228 bp that encode a ubiquitin monomer Five polyubiquitin genes UBQ3, UBQ4, UBQ10, UBQ11, and UBQ14, previous ly isolated from Arabidopsis thaliana ecotype Columbia [10] encode ide ntical mature ubiquitin proteins, but differ in synonymous substitutio ns, nature of amino acids terminating the open reading frame, and in t he number of ubiquitin repeats. The presence of these five genes in ni ne other Arabidopsis ecotypes was Verified by polymerase chain reactio n (PCR). Size differences in UBQ3 and UBell amplified products from se veral ecotypes were observed, suggesting that alleles differ in ubiqui tin repeat number. DNA sequence of UBQ11 alleles from each size class (ecotypes Be-0, Ler, and Rld-0) verified that PCR product size differe nces resulted from changes in the number of ubiquitin repeats. Nucleot ide sequence between two UBell alleles containing the same number of r epeats was identical. Transcript size differences for UBQ3 and UBQ11 m RNAs between ecotypes Columbia and Landsberg indicated that repeat num ber changes did not inactivate these genes. Nucleotide sequence compar isons between UBQ11 repeats from different ecotypes suggest that first repeats are related to each other and last repeats are related to eac h other. We hypothesize that changes in UBell ubiquitin repeat number occurred via the contraction and/or expansion of specific internal rep eats or portions thereof by misalignment of alleles and recombination, most likely via unequal crossing-over events.