An Arabidopsis gene homologous to mammalian and insect genes encoding the largest proteasome subunit.

A gene encoding a protein with extensive homology to the largest subunit of the multicatalytic proteinase complex (proteasome) has been identified in Arabidopsis thaliana. This gene, referred to as AtPSM30, is entirely encompassed within a previously characterized radiation-induced deletion, which may thus provide the first example of a proteasome null mutation in a higher eukaryote. However, the growth rate and fertility of Arabidopsis plants do not appear to be significantly affected by this mutation, even though disruption experiments in yeast have shown that most proteasome subunits are essential. Analysis of mRNA levels in developing seedlings and mature plants indicates that expression of AtPSM30 is differentially regulated during development and is slightly induced in response to stress, as has been observed for proteasome genes in yeast, Drosophila, and mammals. Southern blot analysis indicates that the Arabidopsis genome contains numerous sequences closely related to AtPSM30, consistent with recent reports of at least two other proteasome genes in Arabidopsis. A comparison of the deduced amino acid sequences for all proteasome genes reported to date suggests that multiple proteasome subunits evolved in eukaryotes prior to the divergence of plants and animals.