Wednesday, May 03, 2006

Gene Expression in Prokaryotes

Evolution proposes that an accumulation of gradual changes favored by natural selection produced the biological systems found among living organisms. It's a logical argument. But what do favorable changes "look like" and how do they produce biological systems? The focus of this post will be a particular system known as the lac operon; extensively studied in a prokaryotic organism known as E coli. Evolution is frequently spoken of in abstract terms but some general approaches to evolution on a molecular level can be gleaned from the study of the lac operon.

The lac operon consists of genes that encode for enzymes ß-galactosidase and ß-galactoside permease (lac Z and lac Y). These enzymes enable the lac operon to perform its function of metabolizing lactose to galactose and glucose. Permease is involved in transporting lactose into the cell where ß-galactosidase enables its conversion to galactose and glucose. In addition there are a number of regulatory sites which include a promotor where RNA polymerase binds to initiate transcription and an operator involved in the repression of transcription. Michael Behe describes the lac operon as follows:

"The lac operon of E. coli contains genes coding for several proteins which are involved in metabolism of the disaccharide lactose. One protein of the lac operon, called a permease, imports lactose through the otherwise-impermeable cell membrane. Another protein is an enzyme called β-galactosidase, which can hydrolyze the disaccharide to its two constituent monosaccharides, galactose and glucose, which the cell can then process further. Because lactose is rarely available in the environment, the bacterial cell switches off synthesis of the permease and β-galactosidase to conserve energy until lactose is available. The switch is controlled by another protein called a repressor, whose gene is located next to the operon. Ordinarily the repressor binds to the lac operon, shutting it off by physically interfering with expression of the operon. In the presence of the natural "inducer" allolactose (a by-product of lac β-galactosidase activity) or the artificial chemical inducer isopropylthiogalactoside (IPTG), however, the repressor binds to the inducer and releases the operon, allowing the lac operon enzymes to be synthesized by the cell."1

Evolution has been described as a change in allele frequency but a description of evolution more consistent with what would have actually occurred on earth is: a process wherein genes, associated with a specified biological system produced proteins with new functions whose utility is measured in connection with their interaction with other proteins. All such genes acquired selectivity through coordinated integration of their encoded proteins within a system. Accordingly evidence of evolution on this level requires not only identifying potential precursor proteins but also showing how a sequential integration of them within a system took place. This in turn entails showing how the system retained function while evolving incrementally. Behe's critics have made clumsy attempts at this in their efforts to show how the irreducibly complex examples Behe cited could have evolved. They have identified proteins whose amino acid sequences are similar enough to some of the proteins found in Behe's examples to be considered precursor candidates however, their efforts to document a detailed sequential integration process have fallen far short of the mark. When the lac operon is analyzed in accord with a more realistic description of molecular evolution natural questions arise such as:

1. Is lac function possible without both ß-galactosidase and ß-galactoside permease? If not how was were they integrated into the operon and what was the sequence of events? Does permease have selective value in the absence of a means to metabolize lactose and would a gene encoding ß-galactosidase be selected if permease could not be synthesized?

2. How would the promotor, operator and repressor protein integrate themselves into the regulatory process? What was the sequence of events?

Similar questions arise when examining other biological complexes and the number increases in proportion to the complexity of the systems. Most are more complex than the lac operon. Defenders of evolution often argue that if they can envision or imagine pathways and intermediate functions then Behe's point about irreducible complexity has been adaquately addressed. However Behe's real point is an empirical one. Pathways to irreducibly complex systems are theoretical not observed. As long as this is so Behe's point remains unrefuted.

1. 'A Response to Critics of Darwin's Black Box'; Michael J. Behe

Other references and internet sites for those interested in the lac operon include the following:


Post a Comment

Links to this post:

Create a Link

<< Home