A plant histone gene promoter can direct both replication-dependent and -independent gene expression in transgenic plants.

Chimeric genes containing the beta-glucuronidase (GUS) gene under the control of different Arabidopsis histone H3 and H4 promoters were found to be highly expressed in transient expression experiments using tobacco protoplasts. The activity of one of these promoters, H4A748, was further analyzed. The kinetics of H4A748-GUS activity are very similar to these of a CaMV 35S-GUS constitutive gene during protoplast culture. No increase in H4A748-GUS activity was found after 24 h of protoplast culture when DNA synthesis starts, nor was the GUS activity affected when an inhibitor of DNA synthesis was included in the culture medium. This failure to detect any replication-dependent activity is most likely to be due to the fact that transient transcription of the introduced construct is restricted to the first 24 h following transfection. Stable integration of the H4A748-GUS gene into tobacco plants showed that the histone promoter could confer increased expression in meristematic tissues but it is also expressed to significant levels in non-proliferating tissues. Protoplasts prepared from these transgenic tobacco plants were cultivated under different conditions that affect DNA synthesis. Analysis of H4A748-GUS activity revealed (i) the existence of a basal replication-independent activity and (ii) a replication-dependent activity induced in parallel with DNA synthesis. These results show that the histone H4 promoter is able to direct both replication-dependent and -independent gene expression.