Genomic structure and homoeologous relationship of the two alpha-subunit genes of a heterotrimeric GTP-binding protein in tobacco.

A heterotrimeric GTP-binding protein (G protein) plays a number of important roles in the signal-transduction pathways of eukaryotic cells. The allotetraploid tobacco genome has two alpha-subunit genes, NtGA1 and NtGA2, of the heterotrimeric G protein. In this study, we determined the nucleotide sequences and the exon-intron structures of the NtGA loci in tobacco and its ancestral diploid species. The genomic sequences of the NtGA loci were interrupted by 13 introns. The sizes of most exons (12 of 14) were completely conserved among the NtGA genes and the Arabidopsis alpha-subunit gene (GPA1), but most introns (11 of 13) in the NtGA genes were longer than those in GPA1. In comparison with the genomic sequences of the NtGA orthologues of ancestral Nicotiana sylvestris and Nicotiana tomentosiformis, the tobacco NtGA1 and NtGA2 were concluded to be homoeologous and assigned to the S and T genomes, respectively. More than 300 mutations including insertions-deletions (indels) and nucleotide substitutions were found in the intron regions between the NtGA1 and NtGA2 loci, whereas the exon sequences were highly conserved among these and GPA1. The structural comparison revealed larger divergence at the NtGA2 locus than at NtGA1.