Abstract: The mhcA gene from the human pathogen Entamoeba histolytica was identified using the polymerase chain reaction. It is a single copy gene expressed as a 6.4-kb mRNA. The deduced MhcA protein sequence is highly similar to myosin II from both Dictyostelium discoideum and Acanthamoeba castellanii. The globular head domain of MhcA contains the specific regions involved in ATP binding, actin binding, and interaction with myosin light chain. The tail domain is organized in an alpha-helical coiled coil structure, which suggests that MhcA is an alpha-fibrous protein. The coiled coil is interrupted by two prolines indicating that like other myosins, either from smooth muscle or from non-muscle cells, the tail of MhcA folds twice on itself. In addition, MhcA presents sequence similarities with the heavy chain phosphorylation sites of smooth and non-muscle vertebrate myosins.

Abstract: To recognize myosin II in trophozoites of the human pathogen Entamoeba histolytica, a specific antimyosin polyclonal serum was raised against a fusion protein consisting of a 146-amino-acid fragment of the myosin II heavy chain A of E. histolytica (MhcA) fused with beta-galactosidase. The hybrid protein was encoded by a chimera gene formed by a DNA fragment, from the mhcA gene, amplified by polymerase chain reaction and fused with the lacZ gene of Escherichia coli. Polymerase chain reaction-amplified DNA is located within the region encoding the tail domain of myosin. This antibody recognized a 250-kDa protein in extracts of E. histolytica trophozoites. Confocal microscope analysis of antibody-labelled trophozoites indicated that MhcA localizes at the posterior pole of locomoting cells and concentrates within the uroid. These results might indicate that MhcA is involved in movement and in the uroid formation which help amoebas to escape the host immune response. These data are the first evidence indicating that myosin exists in E. histolytica. In addition, two other peptides were found in myosin-enriched extracts of amoebas, indicating that other myosins may be present in this parasite.

Abstract: The myosin heavy chain gene A, mhcA, from E. histolytica was identified by polymerase chain reaction. There is only one copy of this gene on the ameba genome. The mhcA gene encodes a 6 kb messenger RNA. The predicted protein sequence is homologous to myosin II from both, Dictyostelium discoideum and Acanthamoeba castellanii. Specific areas of the N-terminal region of myosin II are highly conserved reflecting a similarity in functional domains, like the ATP or the actin binding sites.

Abstract: The Bacillus subtilis dinR gene product is homologous to the LexA protein of Escherichia coli and regulates the expression of dinR and dinC. Using transcriptional fusions in the dinR and the recA genes, we have investigated the epistatic relationship between these two genes during the SOS response induced either by DNA damage or by competence. The results show that after DNA damage, induction of the expression of both recA and dinR is dependent on the activity of the DinR and RecA proteins. A RecA-dependent activity on DinR is proposed as the initial event in the induction of the SOS network. In contrast, the competence-related induction of dinR and recA appears to involve two distinct mechanisms. While one mechanism corresponds to the classical regulation of the SOS response, the other appears to involve an activating factor. Moreover, this factor is active in cells in which competence is prevented by a mutation in the regulatory gene comA.

Abstract: A Bacillus subtilis strain deficient in homologous recombination was isolated from a library of Tn917lac insertion mutants. The interrupted locus consists of an open reading frame encoding a 22,823-dalton polypeptide. Analysis of the deduced amino acid sequence revealed 34% identity and 47.3% similarity with the LexA protein from Escherichia coli. The gene was designated dinR. It is located between the recA and thyA genetic markers, at 162 degrees on the B. subtilis chromosome. The dinR gene was shown to be expressed during the entire B. subtilis cellular cycle with at least a threefold increase when cells develop competence. In addition, the use of a merodiploid strain, in which a copy of the wild-type dinR gene coexists with a dinR-lacZ transcriptional fusion, demonstrated that dinR is an SOS gene and that the SOS-induced expression of dinR occurred only when a wild-type copy of dinR was present. In addition, DinR seems to regulate the expression of dinC, another SOS gene.