TRANSCRIPTIONAL PHOTOREGULATION OF CELL-TYPE-PREFERRED EXPRESSION OF MAIZE RBCS-M3 - 3' AND 5' SEQUENCES ARE INVOLVED

In the C4 plant maize, members of the rbcS gene family, encoding the s mall subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, are n ot 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 express ion seen in BSC during the first 24 h that leaves of dark-grown seedli ngs are illuminated requires rbcS-m3 sequences lying between -211 bp a nd +434 bp of the transcription start site. Photoregulated partial sup pression of rbcS-m3 expression in MC, on the other hand, requires gene sequences that lie between -907 bp and -445 bp together with sequence s that lie between +720 and +957 bp within the 3' transcribed region o f the gene. Suppression in MC occurs during the second 24-h period tha t 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 M C has, at the least, a substantial transcriptional component. As repor ted earlier, a converse pattern of suppression in BSC and stimulation of expression in MC is seen in the control of cab-m1 in maize leaves.