Arabidopsis and Brassica comparative genomics: sequence, structure and gene content in the ABI-Rps2-Ck1 chromosomal segment and related regions.

The region corresponding to the ABI1-Rps2-Ck1 segment on chromosome 4 of Arabidopsis thaliana was sequenced in Brassica oleracea. Similar to A. thaliana, the B. oleracea homolog BoRps2 is present in single copy. The B. oleracea orthologous segment was located on chromosome 4 and can be distinguished by the presence of an N-myristoyl transferase coding gene (N-myr) between the Rps2 and Ck1 (BoCk1a) genes. The N-myr homologs in Arabidopsis are on chromosomes 2 and 5. Additional homologs for Ck1 are located on these two chromosomes. A second Ck1 homolog found on B. oleracea (BoCk1b) chromosome 7 served to define another orthologous segment located in Arabidopsis chromosome 1. The two segments displayed identical gene content and order in both species, namely BoCK1b, a gene encoding a hypothetical protein (BohypothA) and transcription factor eiF4A. High levels of sequence identity were observed for the coding sequences of all genes examined. Although in general larger spacers were found in Brassica than in A. thaliana, this was not always the case. Promoters were poorly conserved, except for several sequence stretches of a few nucleotides. Comparative sequencing revealed microsyntenic changes resulting from chromosomal structural rearrangements, which are often undetectable by genetic mapping.