La composition en bases est un facteur dėterminant de la localisation des gnes dans le gėnome. Chez les graminėes, la plupart des gnes sont concentrės dans un intervalle de composition en bases d'environ 2% de GC. Chez Arabidopsis, la composition en GC est plus ėlevėe aux parties distales des chromosomes que dans leurs parties centrales. La composition en GC des gnes et des sėquences codantes suit la mme tendance. Le gėnome d'Arabidopsis peut tre dėcomposė en 2 composantes principales, l'une pauvre en GC correspondant la partie centrale des chromosomes, l'autre riche en GC, et quantitativement plus importante, correspondant aux parties distales des chromosomes. L'expression des gnes associės la composante riche en GC ėtė trouvėe, en moyenne, supėrieure celle des gnes de l'autre composante. La compilation de nos rėsultats et de ceux disponibles dans la littėrature suggre que le plan de base de l'organisation du gėnome est similaire chez les graminėes.
The work shows, by a detailed analysis of the genomic features of Gramineae, that their common ancestor did the experience of a compositional transition leading to the increase in GC level of all parts of the genome and of the genes. By contrast, this transition did not occur in a number of Dicots and Monocots. With regard to GC level of the genes, the compositional transition stresses the existence of two classes of genes in Angiosperms. The GC-poor genes are characterized by introns higher in average number and size compared to GC-rich genes. Moreover, by analysing synonymous and non-synonymous substitutions, we showed that the evolution rates in the two classes of genes are different and correspond to different functional patterns.
The base composition is a determinant of gene location within the Angiosperm genomes. In Gramineae, most genes occur in genomic regions that have a narrow range of GC levels (about 2%). In Arabidopsis, the GC level of contigs is higher, on the average, in the distal regions of chromosomes compared to their central regions. The GC levels of genes and coding sequences follow the same trend. The Arabidopsis genome is made up of two major components. One is GC-poor and corresponds to the central part of the chromosomes, the other, quantitatively higher, is in comparison GC-rich and corresponds to the distal regions of chromosomes. Gene expression is generally higher in this GC-rich component. The sum of our results and those available in the literature suggest that the genome organization of Gramineae is basically similar.