Transcriptional photoregulation of cell-type-preferred expression of maize rbcS-m3: 3' and 5' sequences are involved.

Abstract

In the C4 plant maize, members of the rbcS gene family, encoding the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, are not expressed in mesophyll cells (MC) but are expressed strongly in the adjacent bundle sheath cells (BSC). Expression of genes in an in situ transient expression assay indicates that the photostimulated expression seen in BSC during the first 24 h that leaves of dark-grown seedlings are illuminated requires rbcS-m3 sequences lying between -211 bp and +434 bp of the transcription start site. Photoregulated partial suppression of rbcS-m3 expression in MC, on the other hand, requires gene sequences that lie between -907 bp and -445 bp together with sequences that lie between +720 and +957 bp within the 3' transcribed region ofthe gene. Suppression in MC occurs during the second 24-h period that dark-grown seedlings have been illuminated, but not during the first 24 h. The 3' +720-to +957-bp region is also effective in lowering MC expression when it is relocated to a position >2 kbp upstream of the transcription start site. Thus, suppression of rbcS-m3 expression in MC has, at the least, a substantial transcriptional component. As reported earlier, a converse pattern of suppression in BSC and stimulation of expression in MC is seen in the control of cab-ml in maize leaves.In C4 plants, such (3,4), but within the first 24 h of illumination their abundance increases 2-to 3-fold in BSC and they are undetectable in MC (3, 4). Transcripts of the maize gene rbcS-m3 (5) follow this pattern of BSC-preferred accumulation upon illumination of dark-grown leaves and constitute about 35% of the total leaf rbcS mRNA in 24-h illuminated dark-grown maize (3). Using a 3-glucuronidase (GUS) in situ transient expression assay (6), we found that expression from a reporter gene containing 2.1 kbp upstream of the rbcS-m3 transcription start site [-2.1-kbp rbcS-m3 promoter (Pr):GUS:nopaline synthase (nos) terminator] is about the same in MC and BSC of 24-h illuminated leaves. GUS expression is promoted 2-to 3-fold in BSC by illumination, as expected from the behavior of rbcS-m3, but this reporter gene does not behave like rbcS-m3 in MC after illumination: GUS expression from the rbcS-m3 promoter is about the same in MC of unilluminated and illuminated leaves (3, 6).We have now found that inclusion of a 238-bp sequence from within the transcribed 3' end of rbcS-m3 (extending from +720 to +957 bp relative to the transcription start site) in a reporter gene containing 2.1 kbp of rbcS-m3 from upstream of the transcription start site preferentially reduces expression of GUS in MC in leaves of dark-grown seedlings that are greened (i.e., illuminated) for 24 h prior to bombardment. Furthermore, the 238-bp fragment provides MCpreferred suppression of expression through a transcriptional mechanism. Through a series of promoter deletion experi- (Fig. 1). The pMT211 plasmid was obtained by first digesting pMT444 with Kpn I and HindIII, then treating it with exonuclease III and mung bean nuclease, ...

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

“…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 after dark-grown seedlings have been illuminated; our observations of suppression were made during the second 24 h of illumination (7). Thus, dark-grown seedlings must be illuminated (in our experiments for 24 h) for the rbcS-m3 repression apparatus to develop.…”

“…We have now identified, within the 3Ј ϩ720 to ϩ957 and the 5Ј Ϫ907 to Ϫ445 bp segments of rbcS-m3 (7,9), three regions that are required for repression. The 3Ј region includes a sequence resembling the repressor form of the binding site for the vertebrate transcription activator-repressor YY1 (Yin Yang 1) (10).…”

“…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.…”

Photomorphogenic Responses in Maize Seedling Development

“…These studies suggest that transcription initiation may be a mechanism to regulate cell-specific gene expression. However, in vitro studies also suggest an important role for posttranscriptional control of nuclear gene expression (Viret et al, 1994;Purcell et al, 1995). Our finding that Me1 transcripts accumulated throughout a 72-h time course in D-shifted plants, whereas Ppc and RbcS2 transcripts were undetectable after 48 h, suggests that multiple regulatory mechanisms may mediate the cell-specific patterns of photosynthetic gene expression in maize.…”

“…Transient assays in maize M cells have determined regulatory regions of several C4 photosynthetic genes. For example, promoter fusion constructs transiently introduced into maize leaves have defined several important 5Ј regions necessary for light-regulated, cell-specific gene expression of the RbcS2 gene (Schaffner and Sheen, 1991;Bansal et al, 1992;Viret et al, 1994;Purcell et al, 1995). Studies of the Ppc promoter have also revealed regulatory elements required for proper expression (Schaffner and Sheen, 1992;Kausch et al, 2001).…”