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Pollen, as the male gametophyte, plays a vital role in the reproduction of flowering plants. Mature pollen grains contain mRNAs that are the products of approximately 20,000 different genes, of which up to 4000 may be pollen specific (Mascarenhas, 1990). In recent years, severa1 pollen-specific genes have been isolated from different plants. Some of these genes encode proteins showing amino acid homology to known hydrolytic enzymes, such as polygalacturonase (Allen and Lonsdale, 1993), pectin esterase (Albani et al., 1991), and pectate lyase (Wing et al., 1989). A11 of these genes are expressed after microspore mitosis and their transcripts accumulate considerably in mature pollen. So far, no sequence homology to known proteins has been detected in the other pollen-specific genes studied to date (Hanson et al., 1989). We report here the molecular characterization of another pollen-specific cDNA clone, SF16, from sunflower (Helianthus unnuus L.) (Table I). This gene is present in a single copy in the sunflower genome, expressed late in pollen development, and has no homology to other published sequences. In the original series of northem experiments (not including RNA from pollen), this clone was found to hybridize preferentially to RNA from pistils (Herdenberger et al., 1990); more recent and detailed expression studies revealed that the hybridization was due to contaminating pollen and that the single transcript of 1400 nucleotides can be detected exclusively in mature, free (tricellular) pollen. The transcript was not detected in yellow closed flowers, indicating that it is synthesized at a late stage of pollen development. The cDNA is 1293 nucleotides long with a 993-nucleotidelong open reading frame, starting at the first ATG codon of the cDNA sequence (positions 146-148) and tenninating with a TGA stop codon (positions 1139-1141). Although we have no experimental evidence that the cDNA is full length, the initiation codon can be identified without ambiguity because of the presence of an upstream in-frame stop codon TAA (positions 92-94). Translation of the SF16 cDNA sequence revealed an open

Nucleotide Sequence of a Pollen-Specific cDNA from Helianthus annuus L. Encoding a Highly Basic Protein