Two classes of genes in plants

Two classes of genes were identified in three Gramineae (maize, rice, barle y) and six dicots (Arabidopsis, soybean, pea, tobacco, tomato, potato). One class, the GC-rich class, contained genes with no, or few, short introns. In contrast, the GC-poor class contained genes with numerous, long introns. The similarity of the properties of each class, as present in the genomes of maize and Arabidopsis, is particularly remarkable in view of the fact th at these plants exhibit large differences in genome size, average intron si ze, and DNA base composition. The functional relevance of the two classes o f genes is stressed by (I) the conservation in homologous genes from maize and Arabidopsis not only of the number of introns and of their positions, b ut also of the relative size of concatenated introns; and (2) the existence of two similar classes of genes in vertebrates; interestingly, the differe nces in intron sizes and numbers in genes from the GC-poor and GC-rich clas ses are much more striking in plants than in vertebrates.