A SHORT HISTONE H3 PROMOTER FROM ALFALFA SPECIFIES EXPRESSION IN S-PHASE CELLS AND MERISTEMS

Cell division-dependent activity of a histone H3 promoter from alfalfa (Medicago sativa L.) has been shown through the analysis of both tran sgenic Nicotiana tabacum L. plants and a M. varia L. cell line. A chim eric gene was used to assay the activity of the histone H3 promoter in both transgenic systems. The chimeric gene was composed of 284 bp of 5' upstream sequence from an alfalfa histone H3 genomic clone fused to the beta-glucuronidase coding region and nopaline synthase terminator . The 284 bp of histone H3 promoter sequence specified histochemically detectable beta-glucuronidase expression in the peripheral zone of th e shoot apical meristem and the root tip of transgenic tobacco plants. High beta-glucuronidase activity was as observed in young leaf and ca llus tissues. In protoplast-derived cells, an increased histone H3 pro moter function was detected in 4- to 5-day old cultures and correlated with elevated incorporation of [H-3]thymidine. Northern blot analysis of RNA samples from synchronized cell suspension cultures has reveale d maximum accumulation of both beta-glucuronidase and endogenous histo ne H3 mRNAs in cells at S phase. Whereas the abundance of endogenous h istone H3 transcripts declined abruptly after S-phase, the reporter ge ne showed a more gradual decrease in transcript level after the reduct ion in the number of S-phase cells. The possible role of the short his tone H3 promoter sequence in the regulation of S-phase-dependent expre ssion and the involvement of the 3' end region of histone H3 genes in mRNA stability will be discussed.