Origin of Nicotiana tabacum detected by primary structure of fraction I protein.

Nicotiana tabacum is believed to have arisen after hybridization of Nicotiana sylvestris with a species in the Tomentosae section of the genus Nicotiana. Recent biochemical experiments have confirmed the conclusions from previous cytogenetic studies that N. sylvestris was the maternal parent and have indicated that Nicotiana tomentosiformis was the paternal parent. However, these studies did not take into account the possibility that a new species of Nicotiana, called K-12, discovered in South America in 1968, could also have been one of the parents. Fraction I proteins were purified from N. tabacum and its putative progenitors, and separated into large and small subunits. Chymotryptic peptides of each subunit were analyzed by ion exchange column chromatography with a gradient elution system. Among 38 resolved peaks of the large subunits, 2 peaks were found to be different among the putative species. Since only N. sylvestris showed an identical chromatogram with N. tabacum, N. sylvestris was concluded to be the maternal progenitor, as the genetic information for the large subunit of Fraction I protein was known to be inherited by the cytoplasmic mode. On the other hand, the small subunit of Fraction I protein is inherited by the Mendelian mode and therefore N. tabacum, an allopolyploid, could be expected to contain two types of small subunits, one derived from N. sylvestris and the other from a paternal progenitor. N. sylvestris lacks two of the 25 chymotryptic peptides of the small subunit of N. tabacum. Among 3 putative paternal progenitors, these two peaks appeared only in N. tomentosiformis, but not in Nicotiana otophora or K-12. Thus, N. tomentosiformis was concluded to be a paternal progenitor of N. tabacum. The conclusion was verified by comparing chymotryptic peptides of small subunits from three amphidiploids of N. sylvestris crossed with N. tomentosiformis, N. sylvestris crossed with N. otophora snd N. sylvestris crossed with K-12. The analytical results showed that only the progeny of N. sylvestris crossed with N. tomentosiformis contained the same small subunits as N. tabacium.