In E. coli, the five contiguoustrp genes encode enzymes that sythesize the amino acid tryptophan. These genes are expressed efficiently only when tryptophan is limiting. The genes are controlled by a repressor, just as the lac genes are, although in this case, it is the absence of its ligand (tryptophan) that relieves repression, match trp genes to their gene product,

Trp genes	Gene products
trpE	anthranilate synthetase
trpD	phosphoribosyl anthranilate transferase
trpC	phosphoribosyl anthranilate isomerase-indole glycerol phosphate synthetase
trpB	tryptophan synthetase beta
trpA	tryptophan synthetase alpha

#Question id: 2439

#Unit 2. Cellular Organization

α- and β-tubulin

a. are highly conserved.                                 

b. bind GTP.

c. hydrolyze GTP to GDP.                 

d. are found in all eukaryotes.

e. are related to the bacterial protein FtsZ.

#Question id: 2440

#Unit 2. Cellular Organization

Microtubule assembly in vitro requires

a. ATP.                                                           

b. a threshold (critical) concentration of tubulin.

c. warming the solution to 37°C.                   

d. microtubule-associated proteins.         

e. GTP.

#Question id: 2441

#Unit 2. Cellular Organization

Which of the following statement(s) is (are) true of microtubule assembly?

a. GDP-tubulin normally assembles into microtubules.

b. Microtubule (-) ends grow faster than microtubule (+) ends.

c. Tubulin subunits assemble to form a protofilament.

d. Microtubules consist of multiple protofilaments.

e. Microtubules with a GTP cap will shorten.

#Question id: 2442

#Unit 2. Cellular Organization

The drug colchicine acts to

a. promote microtubule assembly.     

b. inhibit microtubule assembly.

c. sever microtubules.                        

d. block cell division.

e. cap microtubule ends.

#Question id: 2443

#Unit 2. Cellular Organization

Microtubule-associated proteins

a. typically co-localize with microtubules inside cells.

b. are divided into two types: those that stabilize microtubules and those that destabilize microtubules.

c. typically co-purify with microtubules.                  

d. are similar to α- and β-tubulin.

#Question id: 2444

#Unit 2. Cellular Organization

Kinesin I moves vesicles