DNA sequence of the lactose operon: the lacA gene and the transcriptional termination region.

Abstract

The lac operon of Escherichia coli spans approximately 5300 base pairs and includes the lacZ, lacY, and WA genes in addition to the operator, promoter, and transcription termination regions. We report here the sequence of the IacA gene and the region distal to it, confirming the sequence of thiogalactoside transacetylase and completing the sequence of the lac operon. The lacA gene is characterized by use of rare codons, suggesting an origin from a plasmid, transposon, or virus gene. UUG is the translation initiation codon. A preliminary examination of 3' end of the lac messenger in the region distal to 'the lacA gene indicates several endpoints. A predominant one is located at the 3' end of a G+C-rich hairpin structure, which may be involved in termination of transcription or in post-transcriptional processing.An 'open reading frame of 702 base pairs is present on the complementary strand downstream from kacA.Since its description more than 25 years ago, the lactose operon has been a model system of great usefulness in biology. Indeed, study of one or another aspect of the lactose operon has touched on many of the most significant questions of biology. For example, the fundamental question of the mechanisms involved in expression ofgenes was first studied in this system (1). The discovery of the lac repressor and its binding to an operator site on DNA (2, 3) was one of the first problems concerning protein-DNA interactions to be examined. Studies of P-galactosidase in relation to mutants in lacZ have been important in defining many aspects of gene-protein relationships (4). Studies with fragments of f3-galactosidase also have served as a model system for investigating protein-protein interactions (5). The lactose permease, the product of the second structural gene, lacy, was the first membrane transport protein studied extensively (6). Many fundamental concepts ofthe transport of molecules into the cell were derived from these studies. Thiogalactoside transacetylase, which is the gene product of lacA, the third structural gene, has been something of a mystery. The most reasonable interpretation of its function is that it is involved in detoxification (7). As indicated by the experiments of Andrews and Lin (7), cells containing the transacetylase are able to overcome inhibition of growth by thiogalactosides under certain conditions. It is not surprising, therefore, that structures of the components ofthe lactose operon have also been investigated intensively. The amino acid sequence of the lac repressor was determined many years ago (8), and the amino acid sequence ofI-galactosidase was reported in 1977 (9). Both of these proteins were examined by classical methods ofprotein chemistry. When methods for determining DNA sequences became available, the DNA sequence of the control elements of the lactose operon was one of the first to be studied (10).More recently, the DNA sequence of lacZ was determined (11), with the amino acid sequence of f-galactosidase as confirmation (12). The DNA sequence of lac'Y also...

Control of gene expression in bacteriophage P22 by a small antisense RNA. II. Characterization of mutants defective in repression.

“…If the P~ transcript produced in vivo is the same as the Rho-independent, full-length transcript produced in vitro, then s a r RNA has no AUG or GUG codons for translation initiation. The codon UUG, which initiates translation of a few genes in E. coli (Aiba et al 1984;Gren 1984;Hediger 1985), occurs near the 5' end of s a r RNA (see Fig. 2).…”

Characterization of the Gene Encoding Serine Acetyltransferase, a Regulated Enzyme of Cysteine Biosynthesis from the Protist ParasitesEntamoeba histolyticaand Entamoeba dispar

“…First, a structural domain, which is composed of parallel ␤-strands and called a left-handed parallel ␤-helix (47), appears to be well conserved in the amoebic SATs at the primary sequence level. This structure has been found in various acetyl-and acyltransferases, including UDP-N-acetylglucosamine 3-O-acyltransferase (47), chloramphenicol acetyltransferase (48), thiogalactoside acetyltransferase (49), and Rhizobium nodulation protein NodL (50). Second, the amoebic SATs contained a unique insertion between the coil regions 2 and 3.…”

Cells need safety valves

“…With its three genes and its repressor, the lactose operon is presented in all university textbooks. (3) It is also a paradigm for the design of standard biobricks of the SB effort, seen as the paradigm of a logical system whose expression is driven by a Boolean function: lactose AND NOT glucose.…”