COMPARISON OF CLONED GENES PROVIDES EVIDENCE FOR INTERGENOMIC EXCHANGE OF DNA IN THE EVOLUTION OF A TOBACCO GLUCAN ENDO-1,3-BETA-GLUCOSIDASE GENE FAMILY

Two genes for prepro glucan endo-1,3-beta-glucosidase (1,3-beta-glucanase; 1,3-beta-D-glucan glucanohydrolase, EC 3.2.1.39) of tobacco were cloned and their sequences were compared with cDNA clones. Southern analysis indicates that the genomic clones represent genes derived from ancestral parents of tobacco similar to the present day species Nicotiana sylvestris and Nicotiana tomentosiformis, whereas the genes represented by two of the cDNA clones appear to be unique to tobacco. The coding sequences of the genomic clones and cDNA clones differed at < 2.2% of the positions, indicating that the tobacco 1,3-beta-glucanase gene family is highly conserved. Alternating blocks of sequence in the cDNA clones were identical to the coding sequence of the two genomic clones. These results and an analysis of evolutionary distances for nucleotide substitution are consistent with the hypothesis that the evolution of the tobacco 1,3-beta-glucanase gene family has involved exchange of DNA between members of the tomentosiformis and sylvestris subgenomes by recombination or gene conversion.