Reiteration distal

Archaebacterial RNA polymerases are very different from their eubacterial counterparts and more closely resemble eukaryotic enzymes both in their subunit complexity and in their amino acid sequences (for review, see Puehler et al., 1989). This view is also reflected in the diversity of the DNA sequences that are used by the transcription apparatus as signals for initiation of transcription, namely, the promoters. Many attempts were made to identify a consensus promoter structure (Zillig et al., 1988). However, as more genes are isolated and characterized, the picture becomes less coherent. Earlier identification of two upstream sequences, box A and box B, located around positions — 30 and + 1, respectively, gave way to two elements —DPE (distal promoter element) and PPE (proximal promoter element)—located - 38 to - 25 and — 11 to — 2, respectively (Reiter et al., 1990). The DPE encompasses the box A sequence TTTA(A or T)A, but the PPE sequence seems to depend more on an (A + T)-rich sequence rather than on a specific DNA sequence. 

In the truly pendant Weymouthia mollis (Figure 15.3B), the architectural unit consists of a primary module with somewhat distant determinate lateral branches. Reiteration is by distal lateral innovation. There is no stipe development, but there is leaf differentiation between the main axis (primary module) and the lateral branehes (seeondary modules). 

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