Bacterial Growth, Genetics, and Virulence
• If nutrients are available, synthesis of new chromosome begins before previous chromosome synthesis is completed. Once duplication is initiated, the process is completed even when culture conditions (e.g., starvation) are detrimental to the cell.
|Allele||A particular example of a gene. Each variant of a given gene is a different allele of that gene. Genes that are represented by multiple alleles in a population are said to be polymorphic.|
|Cistron||Region of DNA that codes for a single protein; a complementation unit|
|Operator||Nucleotide sequence, located between the promoter and first structural gene of an operon, that binds a repressor protein|
|Operon||Bacterial transcription unit comprising a promoter, operator, and one or more structural genes|
|Plasmid||Small extrachromosomal DNA molecule capable of autonomous replication in bacteria|
|Promoter||Nucleotide sequence in an operon that is recognized by RNA polymerase|
|Replicon||Replication unit, consisting of a replication origin, a replication terminus, and the intervening coding sequence|
7-2 Regulation of the lactose (lac) operon in Escherichia coli. The three structural genes in the lac operon (Z, Y, and A) encode enzymes that metabolize lactose. Transcription yields a single polycistronic messenger RNA (mRNA), which is translated into the three proteins. A, Transcription is directly inhibited by binding of the lac repressor (encoded by the I gene) to the operator. B, In the presence of lactose, the inducer allolactose forms. This molecule binds to the lac repressor, forming a complex that cannot bind to the operator, thereby derepressing the operon. C, Transcription is stimulated by binding of the activator cAMP-CAP to the promoter. In the absence of glucose, the cAMP level increases, permitting formation of cAMP-CAP. Thus, the lac operon is fully “turned on” only when lactose is present and glucose is absent. cAMP, cyclic adenosine monophosphate; CAP,cAMP-binding protein.