SummaryCo.suppreseion of the pigmentation gene chelcone synthese (chs) in Petunia hybrida by chs transgenes leads to white or variegated flowers and is characterized by a reduction in steady-state mRNA levels. To determine the level at which suppression occurs different petunia transformants were analysed containing CaMV-35S RNA promoter-driven hybrid genes consisting of the ~glucuronidase gene (uidA) linked to the full-length chsA cDNA, the 5' half or to the 3' half. With these transgenes one out of 12-15 primary transformants showed suppression of the transgenes and of the resident chs genes throughout the flower or in sectors. The reduction in steady-state chs mRNA was not the result of a transcriptional inactivation event. As determined by nuclear run-on experiments, transcription of suppressed chs genes was similar to that of nonsuppressed genes. This indicates that co.suppression occurs post-transcriptionally. Among individual transformants the transgenes were transcribed at different levels but neither a high nor a low level correlated with a particular degree or pattern of suppression. Surprisingly, even a promoterless chs transgene construct was found to suppress the endogenous chs genes in three out of 15 transformanta. It remains, however, unknown whether or not transcription of the transgene locus is required to induce co-suppression. The data suggest that properties of the chs transgene locus other than the expression level are important for inducing cosuppression. The possible role of antisense RNA, which was detected in all transformants, ectopic pairing and the structure of the integrated T-DNAs in the mechanism of the selective increase in chs RNA turnover are discussed.