Dna

Gene

Transcription mRNA *

Translation

FIGURE 9.02 Efficiency Versus Rapid Response

Two common regulatory mechanisms serve two separate needs. For an efficient response—in other words, one requiring less energy—the gene is often regulated at the level of transcription. When a rapid response is needed, a precursor protein is produced in advance and then rapidly converted to an active protein when the conditions warrant.

Active protein

Inactive protein

Inactive protein

Active protein

Recognition: Obviously many regulatory proteins recognize binding sites on the DNA. Here we are referring to the recognition of the promoter by RNA poly-merase itself. In eukaryotes, there are three different RNA polymerases that recognize and transcribe different categories of genes. In bacteria, there is a single RNA polymerase, but there are multiple different sigma factors.

Initiation: Even if recognition is successful, RNA polymerase may be unable to initiate RNA synthesis. In some cases, activator proteins that bind upstream of RNA polymerase may be needed. In other cases, a repressor that blocks movement of RNA polymerase prevents transcription.

Elongation: Once transcription has been initiated, it usually continues without interruption. Regulatory effects at the stage of elongation are uncommon. They may be subdivided into slowing down of the elongation rate and premature termination.

Termination: Normally, RNA polymerase stops at terminator sites. However, in a few rare cases, termination may be over-ridden by anti-terminator proteins. This allows for the expression of those genes downstream of the terminator and their regulation.

anti-terminator protein Protein that allows transcription to continue through a transcription terminator

Different sigma factors recognize different groups of genes.

TABLE 9.01 Alternative Sigma Factors of Escherichia coli

Sigma factor Name CONSENSUS SEQUENCE

TABLE 9.01 Alternative Sigma Factors of Escherichia coli

Sigma factor Name CONSENSUS SEQUENCE

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