Siberian Inst. Plant Physiol. Biochem.

ORF2 of S mitochondrial plasmids potentially encodes a mammalian type protein kinase

--Yury M. Konstantinov, Miroslava V. Derenko, Vladislav P. Ilyin and Vladimir M. Blinov

Mitochondria of cms-S maize contain linear S1 and S2 plasmids of 6.4 and 5.4kbp (Pring, D et al., Proc. Natl. Acad. Sci. USA 74:2904, 1977). S1 and S2 DNA molecules have proteins covalently attached to their 5' termini that may be involved in replication of these DNAs (Kemble, R et al., Nucl. Acids Res. 10:8181, 1982). The S plasmids also contain a 1462bp region of nearly perfect homology, which includes the ORF2 sequence (Paillard, M et al., EMBO J. 4:1125, 1985). We performed a search for possible homology of protein encoded by ORF2 (327 aa) to Protein Sequence Database "SWISSPROT" (release 17) and found that this protein has sequence homology with the alpha-catalytic subunit of bovine and mouse cAMP-dependent protein kinase (EC (Figure). The amino acid sequence of ORF2 also contains two of seven highly conserved subdomains (II and III) being present in eukaryotic protein kinases (Feng, X-H et al., FEBS Lett. 1:98, 1991). The existence of DNA-dependent protein kinases, which obviously play a significant role in the regulation of DNA transcription and replication, has been shown recently in mammalian cells (Lees-Miller, SP et al., Mol. Cell. Biol. 10:6472, 1990; Lees-Miller et al., J. Biol.

Figure. Sequence homology of ORF2 proteins with bovine and mouse protein kinase. PK: Bovine and mouse cAMP-dependent protein kinase, alpha-catalytic subunit; ORF2: ORF2 of maize mitochondrial linear plasmids S1 and S2.

Chem. 264:17275, 1989). ORF2 is suggested to encode similar protein kinase regulating genetic functions of S1 and S2 episomes of maize through phosphorylation of appropriate proteins. In this case, one of the possible targets of the protein kinase action may be the terminal proteins attached to 5' ends of these minichromosomes.

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