EXPRESSION OF EXTENSIN GENES IS DEPENDENT ON THE STAGE OF THE CELL-CYCLE AND CELL-PROLIFERATION IN SUSPENSION-CULTURED CATHARANTHUS-ROSEUS CELLS

To isolate cDNAs expressed at a specific phase of the cell cycle in a higher plant, we performed differential screening of a cDNA library pr epared from the S-phase cells of synchronized cultures of Catharanthus roseus. Sequence analysis shows that two of the identified cDNAs, cyc 15 and cyc17, encode extensins that represent a family of cell wall hy droxyproline-rich glycoproteins. Protein sequences deduced from the tw o cDNAs contain the characteristic pentapeptide repeat sequence, Ser-P ro-Pro-Pro-Pro, which is commonly observed in extensins. The protein s equences also share several other extensin characteristics such as the presence of a N-terminal signal peptide and a high content of Tyr and Lys residues. When C. roseus cell suspension cultures were synchroniz ed by phosphate starvation, the mRNAs of both cyc15 and cyc17 were tra nsiently expressed during the S and G2 phases of the cell cycle. Howev er, significant amounts of the mRNAs also accumulated in phosphate-sta rved cells arrested in the G1 phase. In asynchronous cultures, both ge nes were expressed during the stationary phase, when cell proliferatio n ceased. The observed patterns of expression suggest that the extensi n genes, cyc15 and cyc17, are under two types of regulation: one that depends on the stage of the cell cycle and another that is induced dur ing the growth arrest. Thus, the products of these genes may function both during the progression through the cell cycle and in the strength ening of the cell wall after cell division.