Nucleotide sequence of two anther-specific cDNAs from sunflower (Helianthus annuus L.)

Abstract

We have recently isolated and characterized several flower-specific clones from a sunflower cDNA library [ 1]. Two of these clones, SF2 and SF18, were shown to hybridize exclusively to RNA from anthers in the late developmental stages (none of the two RNA species is detected in anthers from a closed sunflower inflorescence). The two mRNAs, which are abundant RNAs in anthers from male-fertile plants, are barely detectable in anthers from male-sterile plants. In situ hybridization studies using the labeled SF2 cDNA clone as probe have shown that the SF2 mRNA is present only in the epidermal anther cells (Evrard et al., submitted). The lack of crosshybridization between the two cDNAs suggested that the SF2 and SF18 mRNAs code for different proteins. We describe here the nucleotide sequence of the two cDNAs SF2 and SF18. The sequence analysis revealed a size of 653 bp for SF2 cDNA and 656 bp for SF18 cDNA (Fig. 1); this is a little smaller than that of the corresponding mRNAs estimated in northern experiments (800 nucleotides for SF2 and 750 nucleotides for SF18) and indicates that the two cDNAs are incomplete. Translation of the cDNA sequence of SF2 revealed an open reading frame starting with a methionine and extending over 121 amino acids. That translation must start at this specific methionine residue in vivo has recently been confirmed by the study of the corresponding gene (C. Domon, unpublished). SF18 cDNA carries a 161 amino acid-long reading frame, which is incomplete at the amino terminus. The first 8 amino acids of this incomplete SF18 reading frame are also found in the amino terminal region of SF2 (only one valine substituting for a leucine). In SF2, this region carries a typical signal peptide sequence (21 amino acids, 16 of which are hydrophobic). Not only are the SF2 and SF18 mRNAs similar in size, they also share short segments of significant sequence homology: one of these is found in the region coding for the signal peptide (where 25 out of 28 nucleotides are identical); another one is found in the untranslated 3' region which contains the polyadenylation site (AATAAA) and a short poly(A) stretch (21 A residues in SF2 and 18 A residues in SF18). This region is considered to be involved in mRNA stability [2]. Although no additional sequence similarity can be detected in the SF2 and SF18 polypeptides (except for the signal peptide sequence), they share some features suggesting that they are functionally related. One of these features is a pattern of 7 cysteine residues with a nearly identical spacing in the amino moiety of the two mature polypeptides. This 'cysteine domain' (Fig. 2A), is alsoxich in charged amino acids (35 ~o in SF2 and