Classification and expression of a family of cyclin gene homologues in Brassica napus.

In order to investigate the role of cell division in plant development, we isolated several plant genes which encode homologues of animal and yeast cell cycle regulators known as cyclins. Through the use of degenerate primers and the polymerase chain reaction (PCR) we isolated a Brassica sequence which showed homology to the 'cyclin box' functional domain found within cyclin proteins. Southern blot analysis indicated that Brassica napus has a large number of genes containing cyclin box-related sequences. This was further supported by the isolation of cyclin box sequences from six different genomic clones. In addition, we have isolated two different cyclin cDNA clones, BnCYC1 and BnCYC2, from a Brassica napus shoot apical cDNA library. Both of the cDNA clones contain a 'destruction box' regulatory domain similar to animal mitotic cyclins. Northern blot analysis using BnCYC2 shows mRNA levels which correlate well with the level of cell division in various tissues. Messenger RNA abundance was highest in 1-3 mm leaves, root tips and shoot apices. The mRNA detected using BnCYC1 was restricted to young leaves and the shoot apex, suggesting divergent, organ-specific roles for cyclin family members. The results demonstrate that the plant cyclin gene family is more extensive than previously demonstrated and consists of genes expressed in all dividing tissues as well as a subset of developmentally specific members.