It has previously been found that the sequence and conformation of a 10-nucleotide element in the 5´untranslated region (5´UTR) of rbcL mRNA are important for transcript stability. In this study two variants of a 20-nucleotide sequence were added to the 5´terminus of the rbcL 5´UTR in a chimeric [rbcL 5´UTR: GUS: psaB 3´end] reporter gene construct. The two 5´extensions were predicted to fold into different RNA secondary structures (small and large loops) but did not affect the conformation of the previously identified 10-nucleotide stability element. Addition of the two 20-nucleotide sequences resulted in significantly (? 99%) reduced accumulation of transcripts of the chimeric reporter gene construct suggesting that the extra nucleotides at the RNA´s 5´end rendered the transcripts susceptible to RNase attacks.