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, ...