Structure of a gene encoding a cytosolic acetyl-CoA carboxylase of hexaploid wheat.

An entire gene encoding wheat (var. Hard Red Winter Tam 107) acetyl-CoA carboxylase [ACCase; acetyl-CoA:carbon-dioxide ligase (ADP-forming), EC 6.4.1.2] has been cloned and sequenced. Comparison of the 12-kb genomic sequence with the 7.4-kb cDNA sequence reported previously revealed 29 introns. Within the coding region, the exon sequence is 98% identical to the known wheat cDNA sequence. A second ACCase gene was identified by sequencing fragments of genomic clones that include the first two exons and the first intron. Additional transcripts were detected by 5' and 3' RACE analysis (rapid amplification of cDNA ends). One set of transcripts had a 5' end sequence identical to the cDNA found previously and another set was identical to the gene reported here. The 3' RACE clones fall into four distinguishable sequence sets, bringing the number of ACCase sequences to six. None of these cDNA or genomic clones encodes a chloroplast targeting signal. Identification of six different sequences suggests that either the cytosolic ACCase genes are duplicated in the three chromosome sets in hexaploid wheat or that each of the six alleles of the cytosolic ACCase gene has a readily distinguishable DNA sequence.

Differentiation between homoeologous chromosomes 1A of wheat and 1Am of Triticum monococcum and its recognition by the wheat Ph1 locus.

In most allopolyploid plants, only homogenetic chromosome pairing occurs in meiosis, as a result of the recognition of genome differentiation by the genetic system regulating meiotic chromosome pairing. The nature of differentiation between chromosomes of closely related genomes is examined here by investigating recombination between wheat chromosome 1A and the closely related homoeologous chromosome 1Am of Triticum monococcum. The recognition of the differentiation between these chromosomes by the Ph1 locus, which prevents heterogenetic chromosome pairing in wheat, is also investigated. Chromosomes 1A and 1Am are shown to be colinear, and it is concluded that they are differentiated "substructurally." This substructural differentiation is argued to be recognized by the Ph1 locus. In the absence of Ph1, the distribution and frequencies of crossing over between the 1A and 1Am homoeologues were similar to the distribution and frequencies of crossing over between 1A homologues. The cytogenetic and evolutionary significance of these findings is discussed.