Overlapping genes at the cheA locus of Escherichia coli.

Abstract

The cheA locus of Escherichia coli, which is required for chemotactic behavior, encodes two polep tide products designated p[cheA]L and pjcheA]s. The mode of synthesis of these two proteins was investigated b transferring various missense and nonsense mutations to a A transducing phage and observing the mutant cheA products made after infection of ultraviolet-irradiated host cells. Missense mutations had no effect on either the size or the relative amounts of the two cheA polypeptides. Overlapping genes, which synthesize more than one polypeptide product from the same coding sequence, have been observed in a number of bacterial and animal viruses. Some overlapping gene pairs in bacteriophage-for example, A and A* in 4X174 or C and Nu3 in X-are transcribed from a common promoter and translated in the same reading frame, but from different initiation sites, yielding two related proteins (1, 2). The A1 and coat proteins of phage QB are also translated in the same phase but differ at their carboxyl ends due to an inefficient translation termination signal (3). Other overlapping genes in bacteriophages OX174 and G4 are transcribed from independent promoters and then translated in different reading frames, yielding essentially unrelated products from the same coding sequence (4,5). Animal viruses such as polyoma virus and simian virus 40 utilize messenger RNA splicing to synthesize several proteins, either in the same or different reading frames, from one coding sequence (6, 7).Regardless of their mechanism of expression, overlapping genes are generally thought to have evolved as a means of increasing the coding capacity of viral genomes without a concomitant increase in overall genorne size, which in most viruses is limited by the capacity of the virion. According to this view, the evolutionary constraints that accompany the use of the same nucleotide sequence by two different genes are offset by selective pressures to retain a particular genome size. If this were the only factor favoring the existence of overlapping genes, it seems unlikely they would have arisen in bacteria or higher cells, in which limitations on the amount of genetic material are much less severe than in viruses.In this report we describe an apparent case of overlapping genes at the cheA locus in Escherichia colh. Mutants defective in cheA function are viable but nonchemotactib due to an inability to change directions as they swim (8, 9). Silverman, Simon, and coworkers (10, 11) constructed X phages carrying the cheA region and showed that cheA complementation activity was associated with the presence of two polypeptides.t In order to determine the nature of the relationship between these two cheA products, we examined the functional properties and the polypeptide products of a variety of cheA mutants. Our findings are consistent with a model in which both polypeptides are translated independently from the same coding sequence and in the same reading frame, but from different initiation sites. Moreover, each polypeptide may have a distin...

The carboxy-terminal portion of the CheA kinase mediates regulation of autophosphorylation by transducer and CheW

“…The N-terminal portion is similarly dispensable, as evidenced by the ability of CheAs to form dimers (35,37). This is consistent with the finding that mutations in the central portion of CheA affect protein stability (29,32). A simple explanation for this correlation is that dimer formation provides resistance to proteolysis and that the central domain is involved in dimerization.…”

“…The observation of kinase activity in CheA538R(Am) but not in CheA5OlI(Am) (Fig. 2) (17,32), possesses kinase activity (37).…”

“…Intragenic complementation between mutations in the 5' and 3' portions of the cheA gene suggests the existence of at least two distinct functional domains (32). One such domain has been identified.…”

A second isoform of the ferredoxin:NADP oxidoreductase generated by an in-frame initiation of translation

“…A putative translation start is found at Met-113, which would produce an FNR S with the right size (34 kDa) and an N-terminal sequence matching the one found by Matsuo et al (18) for an FNR that copurifies with the NDH-1 complex in Synechocystis. Although this may sound curious, many examples of internal ribosome entry sites (IRESs) within coding regions have been described in various organisms, although rarely in prokaryotes (26)(27)(28). To test this hypothesis, we introduced missense and frame-shift mutations in petH by a procedure similar to that described in ref.…”

Molecular architecture of chemoreceptor arrays revealed by cryoelectron tomography of Escherichia coli minicells

“…P5 is homologous to CheW and is essential for interaction of CheA with both CheW and the MCPs (12,13). The CheA gene of E. coli encodes two proteins, CheA L (654 aa) and CheA S (557 aa) (14), made by initiating translation from different start codons. CheA S , which lacks all but helix 5 and part of helix 4 of P1, is sufficient to mediate the clustering of MCPs but cannot support chemotaxis on its own (15).…”