Extra stuff on N assimilation

Question 1

PII has a 3rd role:

Answer 1

blocking ammonia overload

ammonia enters the cell through a transporter called AmtB
PII can bind to AmtB and block it when N is high (PII-UMP does not bind)

Question 2

the AmtB channel is blocked by PII only when

Answer 2

the Gln:2oxoglut ratio is high

Question 3

Targets of the global regulator NtrC-P

Answer 3

glnA (GS)
nas (nitrate and nitrite assimilation)
urease (urea>ammonia)
nif (nifLA - N gas)
hut (amino acid recycling)

Question 4

Nitrate NO3- assimilation

Answer 4

nitrate reduced to nitrite by nitrate reductase

Question 5

Nitrite NO2- assimilation

Answer 5

nitrite reduced to ammonia by nitrite reductase

ammonia then captured by GS

Question 6

Global regulation needs to be supplemented by

Answer 6

local regulation

eg. the Hut pathway
NtrC-P is the global regulator
HutC is the local regulator

Question 7

Nitrogenase is the cell’s

Answer 7

last resort
least preferred source of N
metabolically expensive

Question 8

Nitrogenase is the cell’s last resort because

Answer 8

requires lots of ATP
requires >20 genes
easily destroyed by O2

Question 9

Nitrogenase is only expressed when there is

Answer 9

a severe shortage of N and no O2

Question 10

Nitrogenase transcription

Answer 10

NtrC-P activates the transcription of two nitrogenase regulators: nifL and nifA

NifL-ox is an inhibitor of NifA - it is an oxygen sensor
NifL-red does not inhibit NifA

NifA is a TF that activates transcription of the nitrogenase operon

Question 11

How does NifL-ox inhibit NifA?

Answer 11

It binds to NifA

Question 12

How can a cell have a preferred nitrogen source?

Answer 12

by promotor affinity for NtrC-P

gln, ntrB and ntrC have highest affinity
nifL/A has lowest affinity

cells grown in plentiful N will only have a few molecules of ntrc-p in them. subsequently when cells are starved of N, there will only be enought ntrc-p for the genes with the highest affinity binding sites. during a prolonged shortage, ntrc-p will activate the production of more ntrb/c leading to an increase in ntrc-p levels. eventually there will be enough ntrc-p for binding even the lowest affinity binding sites

Question 13

regulation of nitrogenase in symbiotic bacteria is not

Answer 13

the same as other bacteria and is not controlled by NtrC-P. There is NO N check, only a check for O2

FixL sensor kinase detects O2. In the absence of O2, FixL phosphorylates FixJ which turns on nitrogenase genes (NifA and FixK - regulators of nitrogenase genes)

Question 14

Ammonia is the

Answer 14

preferred N source

Question 15

The global control of NtrC-P is subject to local control because

Answer 15

no matter how bad the N shortage, nitrogenase will not be made in the presence of O2