@article{tang_maxwell_2008, title={Xenopus microRNA genes are predominantly located within introns and are diffferentially expressed in adult frog tissues via post-transcriptional regulation}, volume={18}, ISSN={["1549-5469"]}, DOI={10.1101/gr.6539108}, abstractNote={The amphibian Xenopus provides a model organism for investigating microRNA expression during vertebrate embryogenesis and development. Searching available Xenopus genome databases using known human pre-miRNAs as query sequences, more than 300 genes encoding 142 Xenopus tropicalis miRNAs were identified. Analysis of Xenopus tropicalis miRNA genes revealed a predominate positioning within introns of protein-coding and nonprotein-coding RNA Pol II-transcribed genes. MiRNA genes were also located in pre-mRNA exons and positioned intergenically between known protein-coding genes. Many miRNA species were found in multiple locations and in more than one genomic context. MiRNA genes were also clustered throughout the genome, indicating the potential for the cotranscription and coordinate expression of miRNAs located in a given cluster. Northern blot analysis confirmed the expression of many identified miRNAs in both X. tropicalis and X. laevis. Comparison of X. tropicalis and X. laevis blots revealed comparable expression profiles, although several miRNAs exhibited species-specific expression in different tissues. More detailed analysis revealed that for some miRNAs, the tissue-specific expression profile of the pri-miRNA precursor was distinctly different from that of the mature miRNA profile. Differential miRNA precursor processing in both the nucleus and cytoplasm was implicated in the observed tissue-specific differences. These observations indicated that post-transcriptional processing plays an important role in regulating miRNA expression in the amphibian Xenopus.}, number={1}, journal={GENOME RESEARCH}, author={Tang, Guo-Qing and Maxwell, E. Stuart}, year={2008}, month={Jan}, pages={104–112} }
 @article{tang_novitzky_griffin_huber_dewey_2005, title={Oleate desaturase enzymes of soybean: evidence of regulation through differential stability and phosphorylation}, volume={44}, ISSN={["1365-313X"]}, DOI={10.1111/j.1365-313X.2005.02535.x}, abstractNote={SummaryThe endoplasmic reticulum‐associated oleate desaturase FAD2 (1‐acyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine Δ12‐desaturase) is the key enzyme responsible for the production of linoleic acid in non‐photosynthetic tissues of plants. Little is known, however, concerning the post‐transcriptional mechanisms that regulate the activity of this important enzyme. The soybean genome possesses two seed‐specific isoforms of FAD2, designated FAD2‐1A and FAD2‐1B, which differ at only 24 amino acid residues. Expression studies in yeast revealed that the FAD2‐1A isoform is more unstable than FAD2‐1B, particularly when cultures were maintained at elevated growth temperatures. Analysis of chimeric FAD2‐1 constructs led to the identification of two domains that appear to be important in mediating the temperature‐dependent instability of the FAD2‐1A isoform. The enhanced degradation of FAD2‐1A at high growth temperatures was partially abrogated by treating the cultures with the 26S proteasome‐specific inhibitor MG132, and by expressing the FAD2‐1A cDNA in yeast strains devoid of certain ubiquitin‐conjugating activities, suggesting a role for ubiquitination and the 26S proteasome in protein turnover. In addition, phosphorylation state‐specific antipeptide antibodies demonstrated that the Serine‐185 of FAD2‐1 sequences is phosphorylated during soybean seed development. Expression studies of phosphopeptide mimic mutations in yeast suggest that phosphorylation may downregulate enzyme activity. Collectively, the results show that post‐translational regulatory mechanisms are likely to play an important role in modulating FAD2‐1 enzyme activities.}, number={3}, journal={PLANT JOURNAL}, author={Tang, GQ and Novitzky, WP and Griffin, HC and Huber, SC and Dewey, RE}, year={2005}, month={Nov}, pages={433–446} }
 @article{hardin_tang_scholz_holtgraewe_winter_huber_2003, title={Phosphorylation of sucrose synthase at serine 170: occurrence and possible role as a signal for proteolysis}, volume={35}, ISSN={["1365-313X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0141792977&partnerID=MN8TOARS}, DOI={10.1046/j.1365-313X.2003.01831.x}, abstractNote={SummarySequence analysis identified serine 170 (S170) of the maize (Zea mays L.) SUS1 sucrose synthase (SUS) protein as a possible, second phosphorylation site. Maize leaves contained two calcium‐dependent protein kinase activities and a calcium‐independent kinase activity with characteristics of an sucrose non‐fermenting 1 (SNF1)‐related protein kinase. Phosphorylation of the novel S170 and the known serine 15 (S15) site by these protein kinases was determined in peptide substrates and detected in SUS1 protein substrates utilizing sequence‐ and phosphorylation‐specific antibodies. We demonstrate phosphorylation of S170 in vitro and in vivo. The calcium‐dependent protein kinases phosphorylated both S170 and S15, whereas SNF1‐related protein kinase activity was restricted to S15. Calcium‐dependent protein‐kinase‐mediated S170 and S15 phosphorylation kinetics were determined in wild‐type and mutant SUS1 substrates. These analyses revealed that kinase specificity for S170 was threefold lower than that for S15, and that phosphorylation of S170 was stimulated by prior phosphorylation at the S15 site. The SUS‐binding peptides encoded by early nodulin 40 (ENOD40) specifically antagonized S170 phosphorylation in vitro. A model wherein S170 phosphorylation functions as part of a mechanism targeting SUS for proteasome‐mediated degradation is supported by the observations that SUS proteolytic fragments: (i) were detected and possessed relatively high phosphorylated‐S170 (pS170) stoichiometry; (ii) were spatially coincident with proteasome activity within developing leaves; and (iii) co‐sedimented with proteasome activity. In addition, full‐length pS170‐SUS protein was less stable than S170‐SUS in cultured leaf segments and was stabilized by proteasome inhibition. Post‐translational control of SUS protein level through pS170‐promoted proteolysis may explain the specific and significant decrease in SUS abundance that accompanies the sink‐to‐source transition in developing maize leaves.}, number={5}, journal={PLANT JOURNAL}, author={Hardin, SC and Tang, GQ and Scholz, A and Holtgraewe, D and Winter, H and Huber, SC}, year={2003}, month={Sep}, pages={588–603} }