FIG. 4.

Model of the transcriptional organization of the puc operon. Four puc mRNAs detected in RNA blots are shown by horizontal arrows, each of whose thickness suggests its relative abundance. The 2.4-kb transcript encoding all of the puc genes is the primary product of the 5′-most major promoter (bent arrow upstream of pucB), although some transcripts (grey arrow) terminate between pucA and pucC at an imperfect inverted repeat (open stem-loop). The pucBACDE transcript is rapidly cleaved by RNase E at multiple sites (arrowheads) and degraded by 3′-to-5′ exoribonuclease(s) to yield the 0.9-kb pucDE and 0.55-kb pucBA processing products, which lack RNase E recognition sites and are stabilized by stem-loops at their 3′ ends. A minor promoter within pucD (smaller bent arrow) produces the 0.6-kb pucE mRNA. The primary products of both promoters terminate at the predicted rho-independent transcriptional terminator downstream of pucE (closed stem-loop).