The Pivotal Roles of the Plant S-Adenosylmethionine Decarboxylase 5′ Untranslated Leader Sequence in Regulation of Gene Expression at the Transcriptional and Posttranscriptional Levels

Abstract

S-Adenosylmethionine decarboxylase (SAMDC; EC 4.1.1.50) is a key rate-limiting enzyme located in the polyamine biosynthesis pathway. When compared with other organisms, the plant SAMDC genes possess some distinct features because they are devoid of introns in the main open reading frame (ORF) but have an intron(s) in their 5# untranslated leader sequences, in which two overlapping tiny and small upstream ORFs (uORFs) are present. Our results show that the presence of the 5# leader sequence plays important roles in transcriptional and posttranscriptional regulation of SAMDC expression. This sequence may help to keep the transcript of its downstream cistron at a relatively low level and function together with its own promoter in response to external stimuli or internal changes of spermidine and spermine to initiate and regulate SAMDC expression. Under stress and high spermidine or spermine conditions, the tiny uORF shows the same function as its overlapping small uORF, which is involved in translational repression and feedback controlled by polyamines. The presence of introns is necessary for the SAMDC up-regulation process when the internal spermidine level is low. Our results suggest that plants have evolved one network to adjust SAMDC activity through their 5# leader sequences, through which transcriptional regulation is combined with an extensive posttranscriptional control circuit.S-Adenosylmethionine decarboxylase (SAMDC; EC 4.1.1.50) is a key enzyme in higher polyamine (PA) biosynthesis (Janne et al., 1978;Pegg et al., 1988). An understanding of how SAMDC genes are regulated is important for elucidating the molecular basis of PA biosynthesis and the role of PAs in plant growth and development.Plant SAMDC is initially synthesized as an inactive proenzyme and is autocatalytically processed to produce the mature form of the enzyme. This process is very rapid and, unlike the mammalian enzymes, is not regulated by the higher PA precursor putrescine (Put;Xiong et al., 1997). Characterization of SAMDC genes reveals the common feature of a long transcript leader sequence that carries upstream open reading frames (uORFs;Hill and Morris, 1992;Franceschetti et al., 2001). Evidence from several studies indicates that uORFs are involved in translational repression of mammalian SAMDC (Ruan et al., 1996;Pegg et al., 1998;Law et al., 2001). In plants, SAMDCs possess a highly conserved overlapping tiny and small uORFs, consisting of 3 and 52 to 53 codons, respectively (Franceschetti et al., 2001). In all cases, both the tiny and small uORFs are overlapped in such a manner that the last nucleotide A of the tiny uORF stop codon is the first nucleotide of the initiating ATG of the small uORF. The small uORF has also been shown to be responsible for the translational repression of the SAMDC gene (Hanfrey et al., 2002). Apart from the uORFs, the plant SAMDCs are not interrupted by introns through their main ORF but have an intron(s) in their untranslated 5# leader sequences, which is in contrast with SAMDCs from other organisms,...

A Dual Upstream Open Reading Frame-based Autoregulatory Circuit Controlling Polyamine-responsive Translation

“…It is likely that the general translational machinery is the cellular polyamine sensor and that the tiny uORF/small uORF module amplifies and transduces the free polyamine concentration signal that regulates AdoMetDC translation and polyamine biosynthesis. In contrast to the model of AdoMetDC translational regulation presented here, Hu et al (43) have suggested recently that the tiny and small uORFs show the same function in response to polyamines. We note that the level of exogenously added spermine used in their study was 20 -100-fold higher than the physiological levels employed in our study.…”

Arabidopsis ribosomal proteins control developmental programs through translational regulation of auxin response factors

“…1A, Insets). This result suggested that similar to rpl4a, the rpl4d and rpl5a mutations induced the secretion of the vacuolar-targeted CLV3: T7:CTPP BL cargo (20).…”

“…As shown in Fig. 1 A and B, the introduction of the rpl4d (SALK_029203) mutant, which is the closest homolog to RPL4A in Arabidopsis (20), and the rpl5a mutant (SALK_089798) in the Vac2 background caused the reversion of the clv3-2 phenotype; the early termination of floral meristems; and, in some instances (39% for rpl4d and 46% for rpl5a), the formation of pin-like structures (Fig. 1A, Insets).…”

“…Although this regulatory mechanism provides a robust framework to understand auxin regulation mediated by the activation/ inactivation of ARF proteins, it is limited in terms of explaining the emerging role of specific components of the translational machinery that are required for plant development and proper auxin responses (15)(16)(17)(18). Translational control has been described as a regulatory mechanism for multiple plant physiological processes, such as polyamine homeostasis (19)(20)(21), sucrose sensing (22)(23)(24), and anthocyanin biosynthesis (25), but the role of this regulatory mechanism in development is still not well established.…”

Deciphering the Role of various cis-acting regulatory elements in controlling SamDC gene expression in Rice

“…2 Rice shows enormous variation in their susceptibility to abiotic as well as biotic stresses. SamDC is a key enzyme in polyamine biosynthesis 3 and abiotic stress tolerance in rice catalyzing the biosynthesis of spermine and spermidine by providing decarboxylated S-adenosyl methionine as aminopropyl donor. 4 …”