The interactions of microfibril-associated glycoprotein (MAGP)-2 have been investigated with fibrillins and fibrillin-containing microfibrils. Solid phase binding assays were conducted with recombinant fragments covering fibrillin-1 and most of fibrillin-2. MAGP-2, and its structure relative MAGP-1, were found to bind two fragments spanning the N-terminal half of fibrillin-1 and an N-terminal fragment of fibrillin-2. Blocking experiments indicated that MAGP-2 had a binding site(s) close to the N terminus of the fibrillin-1 molecule that was distinct from that for MAGP-1 and an additional, more central binding site(s) that may be shared by the two MAGPs. Immunogold labeling of developing nuchal ligament tissue showed that MAGP-2 had regular covalent and periodic (about 56 nm) association with fibrillin-containing microfibrils of elastic fibers in this tissue. Further analysis of isolated microfibrils indicated that MAGP-2 was attached at two points along the microfibril substructure, "site 1" on the "beads" and "site 2" at the "shoulder" of the interbead region close to where the two "arms" fuse. In contrast, MAGP-1 was located only on the beads. Comparison of the MAGP-2 binding data with known fibrillin epitope maps of the microfibrils showed that site 1 correlated with the N-terminal MAGP-2 binding region, and site 2 correlated with the second, more central, MAGP-2 binding region on the fibrillin-1 molecule. Of particular note, immunolabeling at site 2 was markedly decreased, relative to that at site 1, on extended microfibrils with beadto-bead periods over 90 nm, suggesting that site 2 may move toward the beads when the microfibril is stretched. The study points to MAGP-2 being an integral component of some populations of fibrillin-containing microfibrils. Moreover, the identification of multiple MAGP-binding sequences on fibrillins supports the concept that MAGPs may function as molecular cross-linkers, stabilizing fibrillin monomers in folded conformation within or between the microfibrils, and thus MAGPs may be implicated in the modulation of the elasticity of these structures.

5005Colony pleomorphism, or phase variation, expressed by entomopathogenic bacteria belonging to the genus Xenorhabdus, is an important factor which determines the association of the bacteria with their nematode symbiont and the outcome of infection of susceptible insect larvae by the bacteriumnematode parasitic complex. The mechanism underlying phase variation is unknown. To determine whether RecA-mediated processes are linked to phase variation, the recA gene of Xenorhabdus bovienii was cloned and sequenced. When expressed in a recA-deleted strain of Escherichia coli, the X. bovienii recA clone was able to complement the loss of RecA function. X. bovienii chromosomal recA insertion mutants showed increased sensitivity to UV. Phase 1 forms did not show altered ability to convert to phase 2 and no significant differences in expression of other phase-dependent characteristics, including phospholipase C, haemolysin, protease, antibiotic activity and Congo Red binding, were noted. Furthermore, the LD 50 of the X. bovienii recA insertion mutant for Galleria mellonella larvae was not significantly different from that of wild-type strains. From these data the authors conclude that recA is unlikely to be involved in phase variation, the expression of phasedependent characteristics, or virulence factors involved in killing of susceptible larvae.