--Pam Cooper and Kathy Newton
Another aspect of our work on nuclear-cytoplasmic interactions in teosinte-maize hybrids is the effect of nuclear background on mitochondrial protein synthesis. Mitochondria were isolated from Zea luxurians-maize plants generated by the crossing program described in the previous report, and in organello protein synthesis was carried out by the method of Newton and Walbot (PNAS 82:6879). Mitochondria were pelleted, the proteins were solubilized in boiling Laemmli sample buffer, and then separated on 12-18% linear acrylamide SDS gels. Newly synthesized proteins were visualized by fluorography.
When Zea luxurians (Z.l.) cytoplasm is placed in an A619 nuclear background (Z.l.-A619), a novel 22kD polypeptide is synthesized by the mitochondria. This protein is not synthesized when Z.l. is in a W23 background. Both the F1 hybrids Z.l.-A619 X W23 and Z.l.-W23 X A619 do not synthesize the polypeptide. However, when Z.l.-A619 X W23 was backcrossed by A619, synthesis of the polypeptide was observed in 10 of 15 individual offspring analysed. Taken together, these results suggest that the A619 line possesses a recessive nuclear gene that allows the expression of the protein. We are currently analyzing more individuals from this backcross.
Other inbred lines also possess nuclear genes that control the synthesis of the 22kD polypeptide. The F1 hybrid Z.l.-A619 X B73 does not exhibit the synthesis of the protein, whereas the F1 Z.l.-A619 X Mo17 does.
The functional identity of the polypeptide is unknown. However, we have conducted submitochondrial localization studies using a modification of the method of Day, et al. (Aust. J. Plant Physiol. 12:219), and found the protein to be associated exclusively with the membrane fraction of the mitochondria.
The synthesis of a novel polypeptide by maize plants possessing a particular
nuclear-cytoplasmic constitution is reminiscent of the synthesis of a 13kD
polypeptide by cms-T (Dewey, et al. PNAS 84:5374.). However, we
cannot correlate the presence of the 22kD polypeptide with male sterility
in our interspecific hybrids. Although Z.l.-A619 plants shed less pollen
than do Z.l.-W23 or inbred A619 plants with their original N cytoplasm,
the pollen that is shed is functional.
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