Squences with letters aswell as numbers

At the moment I'm using this kind of sequence to provide a primary key
for some of my tables

create sequence group_seq;
select setval('group_seq', (select max(group_ID) from groups));

However I'd like to create a sequence that has this kind of output

g1
g2
g3
g4
g5

where g doesn't change

and

u1
u2
u3 

where u doesn't change

is there are simple way of doing this?