Alternative transcription initiation sites generate two LCA1 Ca2+-ATPase mRNA transcripts in tomato roots

The tomato LCA1 gene encodes a Ca2+-ATPase and gives rise to two major mRNA transcripts and two distinct protein products of different size in tomato roots. The basis of the transcript size difference was investigated to assess whether the mRNA transcripts encoded distinct protein products. Primer extension and S1 nuclease analysis identified two transcription initiation sites at -72 and -1392 from the start of translation. RNA gel blot analysis of poly(A)+ RNA isolated from phosphate-starved tomato roots using probes designed to domains of the 5'-untranslated region (UTR) or the full-length LCA1 cDNA identified mRNAs of 4.7 and 3.6 kb, corresponding to mRNA originating from transcription initiation sites -1392 and -72, respectively. Screening of a cDNA library derived from phosphate-starved tomato roots yielded three cDNA clones, LCA1A, LCA1B and LCA1C (3.6, 4.5 and 5.1 kb respectively). These cDNAs contain full-length LCA1 mRNA sequence derived from each transcription initiation site, with LCA1C additionally containing an intron of 0.6 kb. Sequence analysis indicated 100% identity between the three size classes of cDNA clones except for the differential 5'-UTR and the unspliced intron. Overall, the results indicate that the two major LCA1 mRNA transcripts are derived by differential transcription initiation and that two of the mRNAs may encode identical protein products, while a third mRNA may correspond to a non-functional truncated protein.

Two plasma membrane H(+)-ATPase genes expressed in guard cells of Vicia faba are also expressed throughout the plant

Guard cells modulate stomatal apertures in response to hormones, metabolic demands and environmental stimuli. The guard cell PM H(+)-ATPases play a critical role in this process by generating the electrochemical gradient to drive solute transport and concomitant water flux. The PM H(+)-ATPase activity is specifically regulated by red and blue light, fungal toxins and auxin. To determine if the unique responsiveness of the guard cell PM H(+)-ATPase is due to the expression of a cell-specific isoform, we amplified by PCR, and cloned portions of PM H(+)-ATPase genes VHA1 and VHA2, which are expressed in guard cell protoplasts (GCP). In situ hybridization to leaf tissue sections indicated that VHA1 and VHA2 genes were expressed in guard cells and mesophyll cells but not in epidermal cells or vascular tissues. Furthermore, a gene-specific quantitative reverse transcription (RT)-PCR detected VHA1 and VHA2 mRNAs in both GCP and mesophyll cell protoplast mRNA as well as in mRNA isolated from roots, leaves, stems and flowers. Thus, two PM H(+)-ATPase genes expressed in guard cells are also expressed in many other tissues and cell types. This suggests that the unique responsiveness of the guard cell PM H(+)-ATPases to environmental stimuli results from cell-specific signal transduction pathways rather than the expression of a cell-specific PM H(+)-ATPase.