Molecular Characterization of Two Brassica napus Pollen-Expressed Genes Encoding Putative Arabinogalactan Proteins

Two highly homologous cDNA clones, Sta 39-3 and Sta 39-4, corresponding to mRNAs highly expressed in Brassica napus cv Westar stamens were isolated by differential screening and characterized. Northern blot and in situ analyses demonstrated that Sta 39-3 and Sta 39-4 transcripts accumulate in pollen following the first pollen mitosis and are abundant at pollen maturity, thus identifying them as "late" pollen genes. Sta 39-3 and Sta 39-4 belong to a small gene family. Their predicted proteins share similarities with those deduced from recently isolated cDNAs encoding arabinogalactan proteins, which include a similar molecular mass (approximately 13 kD); high levels of alanine, proline, serine, and threonine, which are interspersed throughout the protein with no obvious repetitive motif; low levels of cysteine, histidine, tryptophan, and tyrosine; and hydrophobic N- and C-terminal ends. The Sta 39-3 and Sta 39-4 proteins may play a role in pollen germination and tube growth.