Hansjörg Lehnherr CURRENT RESEARCH ACTIVITIES Elucidating the mechanisms which lead to the development of a living organism is one of the most fascinating topic of modern biology. Processes as diverse as the embryonic development of higher eukaryotes, the mating cycle of lower eukaryotes, the division cycle of prokaryotes or the parasitic life cycle of viruses all depend on the accurate expression of the genetic information stored in the genome of the respective organisms. Entire regulatory cascades are required to express the proper functions at the correct time and appropriate level in order to produce structures of the complexity of an infective phage particle, an insect wing or a mammalian limb. Bacteriophages prove to be useful organisms in addressing such questions about regulatory processes. They are easy to propagate, have a short 'generation time' and mutations affecting any phage function are readily available. The lytic life cycles of the well studied bacteriophages l (Hendrix et al., 1983) and T4 (Mathews et al., 1983) are outlined in detail. Many of the molecular mechanisms underlying single regulatory steps are understood or are studied intensively. Insights gained from these prokaryotic systems turned out to be valuable stimuli for investigations on analogous mechanisms in the more complex higher organisms. For example, a helix-turn-helix DNA binding motif, similar to the one found in prokaryotic gene regulatory proteins (Pabo and Sauer, 1984) and first discovered in the Cro repressor protein of phage l (Anderson et al., 1981), is present in the eukaryotic homeodomain proteins (Gehring et al., 1990) Even though the general scheme of the parasitic life cycle is common to most bacteriophages, different classes of phages show clearly distinct ways to regulate the expression of their genetic information. The investigation of phages different from T4 or l thus harbours the potential to extend and deepen our knowledge about the diversity of regulatory mechanisms found in nature. Our current research focuses on the regulation of the lytic growth cycle of bacteriophage P1 (for a review on bacteriophage P1 see Yarmolinsky and Sternberg, 1988). Unlike the cycles of phages like T4 or Mu, the lytic cycle of P1 is not regulated in three distinct steps (early, middle and late), but rather seems to switch directly from an early to a late transcriptional stage. For this purpose bacteriophage P1 harbours unique late promoter sequences expressing genes coding for morphogenetic, lysis control and lysis functions (Guidolin et al., 1989). These promoters share the -10 hexamer with the consensus promoter sequences of the host bacterium Escherichia coli (Hawley and McClure, 1983), but lack any homology to the -35 hexamer of the latter. This lack of homology was proposed to be the reason why the P1 late promoters are not recognized by the E. coli RNA polymerase associated with the housekeeping sigma factor s70 (Guidolin et al., 1989). Highly conserved among the late promoters is a short inverted repeat AAGT t ACTT centered around position -22. A mutational analysis confirmed that this repeat is of functional importance (Lehnherr et al., 1992a), maybe serving as the binding site for a DNA binding activator protein or for a phage specific s factor. A previous genetic analysis identified a single phage-encoded function called lpa (formerly gene 10) to be essential and sufficient for late promoter activation (Lehnherr et al., 1991, 1992b). Several lines of evidence indicate that the protein product of the lpa gene is a DNA-binding activator protein that binds directly to the conserved -22 box within the P1 late promoter sequences and thereby activates transcription. This mode of activation would be novel; so far all known phage-encoded (and most bacterial) activators bind to sequences located upstream of position -35, and many of them contact the a-subunit of RNA polymerase. Only a single activator protein, the regulator of the mercury resistance operon of Tn21, MerR, has been shown to bind between positions -10 and -35, where usually s70 is expected to bind, without blocking transcription. MerR has also been shown to interact directly with s70 and to bend the DNA significantly upon binding of the cofactor Hg(II) (Heltzel et al., 1990; Livrelli et al. 1993; Ansari et al., 1995). Experimental evidence to confirm our hypothesis that Lpa binds directly to the -22 inverted repeat of P1 late promoter sequences and activates transcription via protein-protein contacts to s70 is currently missing. We are therefore working hard to make sure that the former statement will not hold true for much longer.   October 12, 1998 Helle M. Mortensen