[Analysis of sequence and polymorphism of R in S-type CMS maize]

The S-type CMS of maize was reported to be caused by a DNA region designated R in mitochondrial genomes. Through PCR using artificial primers, 1 kb DNA fragments were obtained from mitochondrial DNA of Tangxu and WB cytoplasm. The DNA sequences are almost the same as the sequence in R region of CMS-S. Using the DNA fragment from Tangxu cytoplasm as probe to scan the Mo17 CMS-J gene library, several positive clones appeared. The clone B39 contains a 6.7 kb BamHI fragment with R region. Nucleotide sequence analysis found two orfs including the chimeric orf77. Seventeen lines of maize under Mo17 and 77 nuclear background including N, T, C, S four types of cytoplasm were used for RFLP. Three types of R were discovered in S-type CMS, but not in C group. The R in N group was different from that in S group. T group had only part of R region. The Southern blot of Tangxü and Shuang cytoplasm in R regions showed different ratio of R in different nuclear backgrounds. The heterogeneity of R region suggests that the organization and rearrangement of R controlled by the nuclear background. R region seems to be involved in the mechanism of S group CMS. Varieties of structures of R, the ratio of three R types, different DNA sequences in R region of different CMS might be concerned with instability of S-type CMS.