DNA sequence relationships and origins of acetohydroxy acid synthase genes of Brassica napus

Abstract

The allotetraploid Brassica napus has been shown to contain a five-member multigene family (AHAS1 – 5) of the nuclear-encoded chloroplastic enzyme acetohydroxy acid synthase, three members of which are expressed. AHAS1 and AHAS3 are constitutively expressed while AHAS2 expression is ovule- and seed-specific. By sequence and phylogenetic analyses we show that the AHAS1 and AHAS3 genes are 96–98% similar in the coding region and the adjacent 5′ and 3′ noncoding regions and were derived from a common ancestral crucifer gene. In contrast, the AHAS2 gene shares only about 80% sequence similarity with the AHAS1 and AHAS3 genes, limited to the region coding for the mature peptide and in a short region of the presumptive transit peptide. The AHAS2 gene likely arose by gene duplication of a housekeeping AHAS gene and has acquired characteristics different from other plant housekeeping AHAS genes, perhaps owing to different functional constraints. Key words: acetohydroxy acid synthase, Brassica napus, phylogenetic inference, multigene family, DNA sequence.