RESUMEN
A chimeric mitochondrial DNA (mtDNA) configuration of the cytoplasmic male-sterile (cms) sorghum line IS1112C includes a 321 bp open reading frame designated orf107, encoding a predicted polypeptide product of 11.85 kDa. The open reading frame, similar to several other genes associated with cms, consists of amino-terminal sequences derived from an obligate gene. Unlike other examples to date, however, the carboxy-terminal sequences are highly similar to the carboxy terminus of an open reading frame implicated in cms of rice, orf79. The amino-terminal 31 residues of orf107 are 84% similar to atp9, and the carboxy-terminal 49 residues are 57% identical and 80% similar to the carboxy terminus of orf79. Transcripts of orf107 are edited, with four C-to-U changes that alter amino acids. Sorghum lines partially or fully restored to fertility exhibit a high-efficiency internal-orf107 transcript processing activity, precluding abundant whole-length transcripts, while male-sterile lines exhibit only a trace of the activity. Previous data on the abundance of a 12kDa in organello-synthesized polypeptide in male-sterile versus male-fertile lines are correlated with differential orf107 transcript processing activity of these lines. Examinations of backcross and F2 lines suggest a gametophytic mode of restoration, and indicate that enhanced transcript processing activity is necessary, but not sufficient, to restore full fertility. These novel observations indicate that mitochondrial open reading frames associated with cms in different species can include highly similar motifs, and that fertility restoration could involve a mechanism by which synthesis of a cms-associated gene product may be precluded through internal transcript cleavage.
Asunto(s)
ADN Mitocondrial/genética , ADN de Plantas/genética , Grano Comestible/genética , Secuencia de Aminoácidos , Secuencia de Bases , Mapeo Cromosómico , Clonación Molecular , Cartilla de ADN/genética , Grano Comestible/fisiología , Fertilidad/genética , Genes de Plantas , Datos de Secuencia Molecular , Sistemas de Lectura Abierta , Homología de Secuencia de Aminoácido , Homología de Secuencia de Ácido Nucleico , Transcripción GenéticaRESUMEN
We describe fundamental characteristics of sorghum mitochondrial orf25, urf209, and a related chimeric configuration, orf265/130, which is restricted to the IS1112C source of cytoplasmic male sterility in sorghum. Transcripts of urf209 are edited at ten nucleotides, resulting in nine amino-acid changes predicted from genomic sequences. The cDNA-predicted polypeptide product is 23.6 kDa, while Western blot analyses identify a product of 20k Da. Transcription of urf209 is characterized by one or two transcripts, dependent on nuclear background, but this difference is not related to male fertility status. The orf265/130 chimeric region includes 288 bp 95% identical to sequences 5' to maize T-cytoplasm T-urf13 and atp6, which includes a common transcription initiation site, and terminates with a recombinational event involving urf209. The urf209 similarity extends 189 bp, followed by sequences duplicated 5' to sorghum atp6-2. Sequences immediately 3' to the atp6-2 similarity include a second in-frame start codon, defining orf130. Structural features 5' to orf130 are shared with motifs found 5' to several translated mitochondrial open reading frames. The orf265/orf130 configuration is uniquely transcribed, and transcripts of orf130 exhibit one silent RNA editing event. Transcription in somatic cells is not altered by male fertility status.