Escherichia coil azi mutants, whose growth is resistant to millimolar concentrations of sodium azide, were among the earliest E. coil mutants isolated. Genetic complementation, mapping, and DNA sequence analysis now show that these mutations are alleles of the secA gene, which is essential for protein export across the E. coil plasma membrane. We have found that sodium azide is an extremely rapid and potent inhibitor of protein export in vivo and that azi mutants are more resistant to such inhibition. Furthermore, SecA-dependent in vitro protein translocation and ATPase activities are inhibited by sodium azide, and SecA protein prepared from an azi mutant strain is more resistant to such inhibition. These studies point to the utility ofspecific inhibitors of protein export, such as sodium azide, in facilitating the dissection of the function of individual components of the protein export machinery.

Haemophilus influenzae is nearly unique among facultatively anaerobic bacteria in its absolute requirement for exogenously supplied heme for aerobic growth. In this study, a mutant analysis strategy was used to facilitate identification of H. influenzae cell envelope components involved in the uptake of heme. Chemical mutagenesis was employed to produce a mutant of a nontypeable H. influenzae strain unable to utilize either protein-bound forms of heme or low levels of free heme. This mutant was transformed with a plasmid shuttle vector-based genomic library constructed from the same wild-type nontypeable H. influenzae strain, and a growth selection technique was used to obtain a recombinant clone that could utilize heme. Analysis of the DNA insert in the recombinant plasmid revealed the presence of several open reading frames, one of which encoded a 28-kDa protein with significant similarity to the TonB protein of Escherichia coli. This H. influenzae gene product was able to complement a tonB mutation in E. coli, allowing the E. coli tonB mutant to form single colonies on minimal medium containing vitamin B12. When this H. influenzae gene was inactivated by insertional mutagenesis techniques and introduced into the chromosome of wild-type strains of H. influenzae type b, the resultant transformants lost their abilities to utilize heme and produce invasive disease in an animal model. Genetic restoration of the ability to express this TonB homolog resulted in the simultaneous acquisition of both heme utilization ability and virulence. These results indicate that the H. influenzae TonB protein is required not only for heme utilization by this pathogen in vitro, but also for virulence of H. influenzae type b in an animal model.

The Rous sarcoma virus (RSV) pp6O-src protein was expressed in Saccharomyces cerevisiae cells either from a plasmid vector carrying the v-src gene or in yeast cells containing a single-copy v-src gene chromosomally integrated. In both yeast strains, v-src gene transcription is regulated by the galactose-inducible GAL1Opromoter. Growth in galactose-containing medium resulted in constitutive expression of pp6Ov-src in the integrated strain and transient expression of higher levels of pp6Ov-src in the plasmid-bearing strain. The concentration of pp6o0-s' in the plasmid-bearing strain at its peak of expression was approximately threefold lower than that found in RSV-transformed mammalian cells. pp6O-s'r synthesized in yeast cells was phosphorylated in vivo on sites within the amino and carboxyl halves of the molecule. In immune complex kinase assays, the yeast pp6o-src was autophosphorylated on tyrosine and was able to phosphorylate exogenous substrates such as casein and enolase. The specific activity of pp6Ov-src synthesized in yeast cells was approximately 5-to 10-fold higher than that made in mammalian cells. Induction of pp6O-src caused the death of the plasmid-bearing yeast strain and transient inhibition of growth of the single-copy strain. Concomitantly, this induction resulted in high levels of tyrosine phosphorylation of yeast cell proteins. This indicates that pp6Ov-src functions as a tyrosine-specific phosphotransferase in yeast cells and suggests that hyperphosphorylation of yeast proteins is inimical to cell growth.

Several classes of secA mutants have been isolated which reveal the essential role of this gene product for E. coli cell envelope protein secretion. SecA-dependent, in vitro protein translocation systems have been utilized to show that SecA is an essential, plasma membrane-associated, protein translocation factor, and that SecA's ATPase activity appears to play an essential but as yet undefined role in this process. Cell fractionation studies suggested that SecA protein is in a dynamic state within the cell, occurring in soluble, peripheral, and integral membraneous states. These data have been used to argue that SecA is likely to promote the initial insertion of secretory precursor proteins into the plasma membrane in a manner dependent on ATP hydrolysis. The protein secretion capability of the cell has been shown to translationally regulate secA expression with SecA protein serving as an autogenous repressor, although the exact mechanism and purpose of this regulation need to be defined further.