The effect of ongoing protein synthesis on the steady state levels of Gp63 RNAs in Leishmania chagasi.

G63, the major surface glycoprotein of Leishmania chagasi promastigotes, increases 11-fold in amount as promastigotes grow from logarithmic to stationary phase. Transcripts from three different classes of gp63 genes are differentially expressed during this development (Ramamoorthy, R., Donelson, J. E., Paetz, K. E., Maybodi, M., Roberts, S. P., and Wilson, M. E. (1992) J. Biol. Chem. 267, 1888-1895). We studied the effect of protein synthesis inhibitors on gp63 mRNAs. The steady state level of log class gp63 RNA, expressed primarily in logarithmic phase promastigotes, increased 16.5-fold after incubation in cycloheximide. A similar increase in log gp63 RNAs was caused by inhibitors that block different steps in translation. In contrast, the levels of stationary class gp63 RNA, expressed in stationary phase parasites, and constitutive class gp63 RNA, expressed throughout promastigote growth, increased only 2.3- and 1.5-fold, respectively. The latter was not statistically significant. Nuclear run-on assays showed that the cycloheximide effect was not due to an increased rate of transcription. However, the t1/2 of log RNAs was prolonged 6.5-fold after incubation in cycloheximide, in contrast to a 1.7-fold increase in the t1/2 of ATPase RNA, suggesting that cycloheximide specifically stabilizes log gp63 mRNAs. Thus, a highly labile negative regulatory protein, such as an RNase, may specifically target log gp63 RNAs for degradation.