Characterization of a tobacco gene encoding a pollen-specific polygalacturonase

Abstract

We report here the isolation and characterization of a gene which is specifically expressed during late pollen development in Nicotiana tabacum L. cv. Havana and which exhibits homology to bacterial, fungal and plant polygalacturonases. This gene is ca. 4.3 kb, from the transcription start-site to the 3' polyadenylation-site sequences. It contains three introns of 620, 706 and 1400 bp and encodes a 1.5 kb message that contains an A-rich 5'-untranslated-leader sequence of 81 bases and a variable-length 3'-untranslated sequence of between 180 and 320 bases. Located within intron 3 is a 414 bp sequence which exhibits 79% homology to a sequence within the endochitinase gene; both sequences share the same internal repeat structure and exhibit features consistent with them being defective transposable elements. The predicted protein sequence coded for by Npg1 shows, in addition to a number of highly conserved cysteines, four conserved domains with the bacterial and fungal polygalacturonase genes. The pollen-specific polygalacturonases as a group can be distinguished from the fruit-ripening polygalacturonases by a number of criteria. It is suggested that these differences reflect the functional differences between plant endo- and exo-polygalacturonases. Npg1 is one of a two-member gene family expressed predominantly in the male gametophyte upon first microspore mitosis. From expression studies of promoter::GUS transgenes it is clear that the -744 bp to +74/+85 bp of Npg1 sequence (with respect to the transcription start site) is sufficient to drive the expression of the GUS reporter gene in a manner that reflects the spatial and temporal expression of Npg1 as determined by dot-blot and northern analysis.