Degradation of pufLMX mRNA in Rhodobacter capsulatus is initiated by nonrandom endonucleolytic cleavage.

Differential expression of genes within the puf photosynthesis operon of Rhodobacter capsulatus is achieved primarily through marked segmental differences in stability within the polycistronic puf operon transcripts. The comparatively stable pufBA segment of these transcripts outlives the labile pufLMX segment and accumulates as an abundant puf mRNA degradation intermediate. Here we present further evidence that degradation of pufBALMX mRNA is initiated by endonucleolytic cleavage within the short-lived pufLMX mRNA segment. By deletion analysis, a region sufficient to mediate rapid degradation of this labile RNA segment has been defined. The 3' boundary of this region maps to within a stretch of 30 nucleotides corresponding to pufL codons 49 through 59. Evidence that initial cleavage of the pufLMX RNA segment occurs predominantly upstream of pufM codon 99 has been obtained by using a novel method, hairpin insertion analysis. Additional data indicate that the efficacy of RNA stem-loop structures as 3'-exonuclease barriers is reduced when they are located in translated regions of messages.