POLYMORPHIC QUATERNARY ORGANIZATION OF THE BACILLUS-SUBTILIS BACTERIOPHAGE SPP1 REPLICATIVE HELICASE (G40P)

The Bacillus subtilis bacteriophage SPP1 gene 40 product (G40P), which belongs to the DnaB-like family of helicases, is essential for SPP1 g enome replication. The active form of the enzyme is the hexamer, capab le of DNA unwinding with a 5' to 3' polarity fueled by the hydrolysis of a nucleoside 5'-triphosphate. We have used electron microscopy of n egatively stained G40P samples and image processing techniques to stud y the structural characteristics of the hexameric assemblies of this p rotein. Our results provide the first low resolution data on a hexamer ic helicase of a Cram-positive bacterial origin. A novel approach has been adopted to analyze possible symmetry heterogeneities, an unsuperv ised method based on a neural network self-organizing algorithm, which has led to the detection of different subclasses of G40P views. Two d ifferent quaternary states of G40P homohexamers sharing a C-3 symmetry organization have been found, as well as a minor class that seems to reflect an alternative C-6 symmetry architecture. These forms show gen eral features known for other hexameric helicases, such as the ring-li ke arrangement of monomers around a central hole. A clear structural h andedness has also been detected in some of these forms. An analysis o f these quaternary states and a model for the structural organization of G40P are presented. (C) 1998 Academic Press.