STRUCTURE AND EVOLUTION OF GENES ENCODING POLYUBIQUITIN AND UBIQUITIN-LIKE PROTEINS IN ARABIDOPSIS-THALIANA ECOTYPE COLUMBIA

The Arabidopsis thaliana ecotype Columbia ubiquitin gene family consis ts of 14 members that can be divided into three types of ubiquitin gen es; polyubiquitin genes, ubiquitin-like genes and ubiquitin extension genes. The isolation and characterization of eight ubiquitin sequences , consisting of four polyubiquitin genes and four ubiquitin-like genes , are described here, and their relationships to each other and to pre viously identified Arabidopsis ubiquitin genes were analyzed. The poly ubiquitin genes, UBQ3, UBQ10, UBQ11 and UBQ14, contain tandem repeats of the 228-bp ubiquitin coding region. Together with a previously desc ribed polyubiquitin gene, UBQ4, they differ in synonymous substitution s, number of ubiquitin coding regions, number and nature of nonubiquit in G-terminal amino acid(s) and chromosomal location, dividing into tw o subtypes; the UBQ3/UBQ4 and UBQ10/UBQ11/UBQ14 subtypes. Ubiquitin-li ke genes, UBQ7, UBQ8, UBQ9 and UBQ12, also contain tandem repeats of t he ubiquitin coding region, but at least one repeat per gene encodes a protein with amino acid substitutions. Nucleotide comparisons, K-s va lue determinations and neighbor-joining analyses were employed to dete rmine intra- and intergenic relationships. In general, the rate of syn onymous substitution is too high to discern related repeats. Specific exceptions provide insight into gene relationships. The observed nucle otide relationships are consistent with previously described models in volving gene duplications followed by both unequal crossing-over and g ene conversion events.