A Plant Plasma Membrane Proton-ATPase Gene Is Regulated by Development and Environment and Shows Signs of a Translational Regulation

Abstract

A proton-pumping ATPase is present in the plasma membrane of plant cells where it sustains transport-related functions. This enzyme is encoded by a family of genes that shows signs of both transcriptional and post-transcriptional regulation. The regulation of pma1, one of the Nicotiana plumbaginifolia H+-ATPase genes, was characterized with the help of the beta-glucuronidase (gusA) receptor gene in transgenic plants. pma1 is active in the root epidermis, the stem cortex, and guard cells. This activity depends on developmental and growth conditions. For instance, pma1 activity in guard cells was strongly enhanced when the plant material (young seedlings or mature leaves) was incubated in liquid growth medium. pma1 is also expressed in several tissues of the reproductive organs where active transport is thought to occur but where scarcely any ATPase activity has been identified, namely in the tapetum, the pollen, the transmitting tissue, and the ovules. Several pma genes have a long 5'untranslated region (leader sequence) containing an upstream open reading frame (URF). Analysis of translational and transcriptional fusions with gusA in transgenic plants suggests that the pma1 leader sequence might activate translation of the main open reading frame, even though the URF is translated by a large majority of the scanning ribosomes. As confirmation, transient expression experiments showed that the pma1 leader causes a fourfold post-transcriptional increase of main open reading frame expression. Deletion of the URF by site-directed mutagenesis stimulated the main open reading frame translation 2.7-fold in an in vitro translational assay. These results are consistent with a regulatory mechanism involving translation reinitiation. Altogether, they suggest a fine, multilevel regulation of H+-ATPase activity in the plant.

Plant Cell Responses to Arbuscular Mycorrhizal Fungi: Getting to the Roots of the Symbiosis

“…In nonmycorrhizal roots, immunologically detectable plasma membrane H+-ATPase is located in the root cap, epidermis, and stele, but it is not present in cells of the cortical parenchyma (Parets-Soler et al, 1990;Stenz et ai., 1993;Bouché-Pillon et al, 1994). This distribution has been corroborated by studies of the expression of two H+-ATPase genes in promoter-P-glucuronidase (GUS) fusion assays: plasma membrane H+-ATPase promoters are apparently not active in differentiated root cortical parenchyma (DeWitt et al, 1991;Michelet et al, 1994). There is evidence that root plasma membrane H+-ATPase genes are upregulated during AM interactions.…”

Guard Cell Inward K+ Channel Activity inArabidopsis Involves Expression of the Twin Channel Subunits KAT1 and KAT2

“…We propose that the presence of the two genes supports channel expression control and/or generates diversity in regulatory mechanisms modulating channel function properties. In relation to this hypothesis, the length of the KAT2 leader and the presence of several upstream open reading frames strongly suggest translational regulation (28,29). Also, the region that lies between the cyclic nucleotide monophosphate-binding domain and the K HA domain in both channels displays the lowest level of sequence similarities.…”

Translation of an Uncapped mRNA Involves Scanning

“…In instances where the two ORFs overlap, it has been proposed that ribosomes terminate at the end of the upstream ORF then migrate backward to the initiation codon of the downstream ORF where they reinitiate (9 -11). This 3Ј to 5Ј movement, here called "backscanning," may not be universal, since translation of some downstream ORFs is abolished by the introduction of an overlapping upstream ORF (12)(13)(14). An alternative explanation for this phenomenon is that ribosomes paused at the termination codon of the upstream ORF may slow the progress of following ribosomes that have bypassed the upstream AUG, thereby facilitating their initiation at the downstream AUG.…”