TRM1, a YY1-like suppressor of rbcS-m3 expression in maize mesophyll cells

Abstract

The genes rbcS and rbcL encode, respectively, the small and large subunits of the photosynthetic carbon dioxide fixation enzyme ribulose bisphosphate carboxylase͞oxygenase. There is a single rbcL gene in each chloroplast chromosome; a family of rbcS genes is located in the nuclear genome. These two genes are not ex- L eaves of C3 plants contain a single type of photosynthetic cell. Each of these cells carries on all of the reactions of oxygenic photosynthesis and contains the carbon dioxide-fixing enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase͞ oxygenase). This enzyme catalyzes the fixation of CO 2 to ribulose-1,5-phosphate and the production of two molecules of 3-phosphoglycerate.Maize is a C4 plant. Its leaves have two types of major photosynthetic cells: a cylinder of bundle sheath cells (BSC) surrounds each of the vascular bundles; mesophyll cells (MC) occupy the remainder of the space between the upper epidermis and the lower epidermis. Some MC and BSC are immediately adjacent to one another. In MC, CO 2 is fixed to phosphoenolpyruvate (PEP) by PEP carboxylase to form oxaloacetate, the latter, a four-carbon acid, is reduced to malate, which is transferred to BSC. CO 2 is released from the malate in BSC. Rubisco, which refixes the CO 2 , is present in BSC but not in MC. C4 species differ with regard to the exact product of oxaloactate that is moved from MC to BSC but in all cases Rubisco is found only in one cell type.Rubisco is comprised of eight large subunits, each of about 55 kDa, and eight small subunits, each of about 13 kDa. The large subunit is the product of a single chloroplast rbcL gene per chloroplast chromosome (1, 2). The small subunits are encoded by a family of nuclear rbcS genes (3).Transcripts of rbcS and rbcL are detectable in MC and BSC in leaves of dark-grown maize seedlings. Upon illumination the transcripts increase 2-to 3-fold in abundance in BSC but become undetectable in MC (3-5). The maize nuclear gene rbcS-m3 (6) follows the general pattern of rbcS expression in maize, i.e., expression is induced in BSC and repressed in MC upon illumination of dark-grown seedlings. About 35% of the total leaf rbcS mRNA in 24 h illuminated dark-grown maize is transcribed from rbcS-m3 (3). The control of repression of this gene in MC is the subject of the present work.Using an in situ reporter gene transient expression assay, we found previously that sequences of rbcS-m3 that lie between Ϫ93 bp and ϩ64 bp of the transcription start site are required for promoting photoregulated expression in BSC (7,8). On the other hand, photoregulated partial suppression of rbcS-m3-reporter gene expression in MC was found to require gene sequences that lie between Ϫ907 and Ϫ445 bp together with sequences that lie between ϩ720 bp and ϩ957 bp. The latter are just beyond the translation stop codon within the 3Ј transcribed region of the gene, but are equally effective when relocated 5Ј to the Ϫ907 to Ϫ445 corepressor-containing region. We also found that expression of the reporter gene is suppressed in MC only ...

Ectopic Expression of Rubisco Subunits in Maize Mesophyll Cells Does Not Overcome Barriers to Cell Type-Specific Accumulation

“…Our results also are in contrast with those for ME, whose BS-specific cis-element lies within the coding region (Brown et al, 2011). Clearly, C4-specific RNA patterns arise through multiple mechanisms, as regulatory targets for transcriptional or posttranscriptional regulation have been found in the promoter region, such as the MEM1 element in the Flaveria PEPC promoter (Gowik et al, 2004), 59 UTR regions of the amaranth and Flaveria RBCS genes (Patel et al, 2004(Patel et al, , 2006, and also the coding region (ME; Brown et al, 2011) and 39 UTR (RBCS; Xu et al, 2001). …”

“…Early transient expression assays showed that RBCS promoter sequences alone did not confer cell type-specific expression (Bansal et al, 1992), and a 39 UTR element was subsequently found to be important for transcriptional repression in M cells (Viret et al, 1994). Repression requires sequence elements that bind the zinc finger protein TRM1 (Xu et al, 2001). Maize RBCS also appears to be regulated posttranscriptionally, and examination of RBCS cell typespecific expression in other C4 species also suggests both transcriptional and posttranscriptional mechanisms (for review, see Hibberd and Covshoff, 2010).…”

“…Transient expression assays revealed that both promoter and 39 untranslated region (UTR) elements confer this specificity (Viret et al, 1994), and a stably transformed maize transgene consisting of the RBCS promoter, 59 UTR, transit peptide, and 39 UTR, fused to a maize codon-optimized yellow fluorescent protein (YFP) coding region, is expressed in BS but not M chloroplasts (Sattarzadeh et al, 2010). Both the 59 and 39 UTRs of one RBCS family member, RBCS-m3, have binding sites for Transcription Repressor-Maize1 (TRM1), a zinc-finger protein that may repress RBCS expression in M cells, although TRM1 expression itself does not appear to be cell type specific (Xu et al, 2001).…”

C4 Cycles: Past, Present, and Future Research on C4 Photosynthesis

“…Putative trans-regulators of cell-specific gene expression in maize have been identified by gel retardation assays with 59 promoter sequences of genes encoding PEPCase (Taniguchi et al, 2000), Rubisco small subunit (Xu et al, 2001), and PPdK (Matsuoka and Numazawa, 1991). However, the context in which these proteins act is not understood, and the properties of the proteins are not known.…”

Photomorphogenic Responses in Maize Seedling Development

“…Studies of the Ppc promoter have also revealed regulatory elements required for proper expression (Schaffner and Sheen, 1992;Kausch et al, 2001). Furthermore, nuclear-encoded transcription factors have been identified that bind specific regions of the RbcS promoter to repress M cell-specific expression (Xu et al, 2001). Transcriptional activators for Ppc also have been identified through in vitro-binding assays that may control light-and cell-specific patterns of expression (Morishima, 1998;Yanagisawa and Sheen, 1998;Yanagisawa, 2000).…”