RFLP map position of the casein kinase 2 (CK-2) m subunit in maize
--Hanten, J; Edwards, M; Warner, T; Boldyreff, B and Issinger, O-G

Casein kinase 2 (CK-2) is a ubiquitous and multifunctional serine/threonine specific protein kinase that has been implicated in the control of cell growth and proliferation. CK-2 has been characterized extensively in animals and has been shown to phosphorylate various protein substrates including RNA polymerases, topisomerases, oncoproteins, and certain receptor proteins. In plants, less is known about the substrates of CK-2. The subunit composition of CK-2 in animals is m, m', and p with molecular weights of 42, 38, and 28 kDa, respectively. The holoenzyme is comprised of a tetramer consisting of m, m', and p2. Two CK-2-like enzymes have been isolated in maize, CKIIA and CKIIB, with reported molecular weights of 135 and 39 kDa respectively (Dobrowolska et al., Eur. J. Biochem. 204:299-303, 1992).

To confirm the presence of a CK-2 m gene in maize, a maize cDNA library was screened with oligonucleotide probes specific for conserved regions of the animal CK-2 m. A clone was isolated which exhibited a 75% protein sequence homology to the human CK-2 m (Dobrowolska et al., BBA 1129:139-140, 1991). This clone was expressed in Escherichia coli with a reported molecular weight of 39 kDa and designated as recombinant maize CK-2 m (rmCK-2 m) (Boldyreff et al., BBA 1173:32-38, 1993). This work has demonstrated that the rmCK-2 m is functionally similar to the recombinant human CK-2 m (rhCK-2 m) in several respects. First, the rmCK-2 m was shown to be immunologically similar to the rhCK-2 m by western analysis with affinity purified polyclonal and monoclonal anti-human CK-2 m antibodies. Second, the rmCK-2 m self assembles with the rhCK-2 p to form a complex which sediments at the same position as the native mammalian CK-2 holoenzyme. Third, similar phosphorylation profiles are exhibited between rmCK-2 m and rhCK-2 m when different substrates and various polyamines are assayed.

Maize CK-2 m was characterized in a RFLP mapping population to establish its chromosomal map position. It was anticipated that this information would be useful in determining linkage and homology among other maize casein kinase-like genes as they become mapped. An 896bp portion of the maize CK-2 m open reading frame was mapped by RFLP in a F2 population with 200 individuals using 68 polymorphic markers spread over the genome. It was determined with MAPMAKER IBM version 3.0b that maize CK-2 m was located on the long arm of chromosome 2 (2L) approximately 9.6 centimorgans (cM) distal from umc36. In a selfed population with 300 individuals using 108 polymorphic markers, the maize CK-2 m probe mapped again on chromosome 2L, 4.9 cM distal from umc36. A secondary polymorphism mapped to the short arm of chromosome 4 (4S), 4.5 cM proximal to bnl5.46. It is not clear what degree of homology exists at chromosome 4S, but it is possible that this secondary sequence arose via chromosomal duplication, a well characterized feature of the maize genome (Helentjaris et al., Genetics 118:353-363, 1988). It is also possible that another casein kinase-like enzyme or a distinctly different enzyme within the maize genome share homology with maize CK-2 m. 

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