Question

Please provide brief (one sentence) answers:

a) The Mycoplasma chromosome codes for 500 genes, yet it has about 20 promoters. Provide a brief explanation for how this is possible.

b) How many seconds would it take the bacterial RNA polymerase to synthesize an mRNA of 5,000 bases?

c) How many mistakes would the bacterial RNA polymerase make while synthesizing an mRNA of 30,000 bases?

d) Sigma (?) protein by itself does not bind to promoter sites. Predict the effect on transcription of a mutation enabling ? to bind tightly to the –35 region in the absence of other subunits of RNA polymerase.

e) What is the structure of the most common transcription terminator signal in E. coli, and what are 2 conserved features of the RNA sequence of this structure?

f) Why is RNA synthesis not as carefully monitored for errors, as is DNA synthesis?

Answer 1

A. It is given that Mycoplasma contains only 20 promoters but 500 genes. This can be possible if multiple genes are under the regulation of a common promoter as in the case of operons.

B. The elongation rate of RNA polymerase = ?50 ntd/sec
Time required to synthesize 5000 ntd mRNA = 5000/50 = 100 sec

C. The error rate of RNA polymerase = 10^-4
Size of mRNA = 30000 ntd
Number of mutations = 30000 X 10^-4 = 3

D. The mutant sigma protein prevents the binding of the holoenzyme and specific initiation of transcription.
E. The common transcription terminator signal in E coli = Hairpin loop. Termination is facilitated by rho factor.
F. Replication produces DNA which is inherited through generations. Mutations in DNA result in mutant RNA and mutant proteins. Hence mutations in DNA are not tolerated. On the other hand, mutations during transcription will affect only that transcript.