apt-get update public key unavailable error

root@ubuntu1204:~# apt-get upgrade --simulate
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
root@ubuntu1204:~# apt-get update --simulate
Ign http://extras.ubuntu.com precise InRelease
Ign http://in.archive.ubuntu.com precise InRelease
Get:1 http://extras.ubuntu.com precise Release.gpg [72 B]
Hit http://in.archive.ubuntu.com precise Release.gpg
Hit http://in.archive.ubuntu.com precise Release
Hit http://extras.ubuntu.com precise Release
Ign http://extras.ubuntu.com precise Release
Hit http://in.archive.ubuntu.com precise/main Sources
Ign http://extras.ubuntu.com precise/main Sources/DiffIndex
Ign http://archive.canonical.com precise InRelease
Hit http://in.archive.ubuntu.com precise/restricted Sources
Ign http://extras.ubuntu.com precise/main i386 Packages/DiffIndex
Hit http://in.archive.ubuntu.com precise/universe Sources
Ign http://extras.ubuntu.com precise/main TranslationIndex
Hit http://in.archive.ubuntu.com precise/multiverse Sources
Hit http://extras.ubuntu.com precise/main Sources
Hit http://in.archive.ubuntu.com precise/main i386 Packages
Hit http://archive.canonical.com precise Release.gpg
Hit http://extras.ubuntu.com precise/main i386 Packages
Hit http://in.archive.ubuntu.com precise/restricted i386 Packages
Hit http://in.archive.ubuntu.com precise/universe i386 Packages
Hit http://in.archive.ubuntu.com precise/multiverse i386 Packages
Hit http://archive.canonical.com precise Release
Hit http://in.archive.ubuntu.com precise/main TranslationIndex
Hit http://in.archive.ubuntu.com precise/multiverse TranslationIndex
Hit http://in.archive.ubuntu.com precise/restricted TranslationIndex
Hit http://in.archive.ubuntu.com precise/universe TranslationIndex
Hit http://archive.canonical.com precise/partner Sources
Hit http://in.archive.ubuntu.com precise/main Translation-en
Hit http://in.archive.ubuntu.com precise/multiverse Translation-en
Ign http://extras.ubuntu.com precise/main Translation-en_US
Hit http://in.archive.ubuntu.com precise/restricted Translation-en
Hit http://archive.canonical.com precise/partner i386 Packages
Ign http://extras.ubuntu.com precise/main Translation-en
Hit http://in.archive.ubuntu.com precise/universe Translation-en
Ign http://archive.canonical.com precise/partner TranslationIndex
Ign http://archive.canonical.com precise/partner Translation-en_US
Ign http://archive.canonical.com precise/partner Translation-en
Fetched 72 B in 8s (8 B/s)
Reading package lists... Done
W: GPG error: http://extras.ubuntu.com precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126D3A3E5C1192

Now follow and execute the gpg commands by fetching the key from the ubuntu.

root@ubuntu1204:~# gpg --keyserver keyserver.ubuntu.com --recv 3E5C1192
gpg: directory `/root/.gnupg' created
gpg: new configuration file `/root/.gnupg/gpg.conf' created
gpg: WARNING: options in `/root/.gnupg/gpg.conf' are not yet active during this run
gpg: keyring `/root/.gnupg/secring.gpg' created
gpg: keyring `/root/.gnupg/pubring.gpg' created
gpg: requesting key 3E5C1192 from hkp server keyserver.ubuntu.com
gpg: /root/.gnupg/trustdb.gpg: trustdb created
gpg: key 3E5C1192: public key "Ubuntu Extras Archive Automatic Signing Key <ftpmaster@ubuntu.com>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1
root@ubuntu1204:~# gpg --export --armor 3E5C1192 | apt-key add -
OK

Now do an update.. and we are fine.
root@ubuntu1204:~# apt-get update --simulate
Ign http://extras.ubuntu.com precise InRelease
Get:1 http://extras.ubuntu.com precise Release.gpg [72 B]
Ign http://in.archive.ubuntu.com precise InRelease
Hit http://extras.ubuntu.com precise Release
Ign http://archive.canonical.com precise InRelease
Hit http://extras.ubuntu.com precise/main Sources
Hit http://in.archive.ubuntu.com precise Release.gpg
Hit http://extras.ubuntu.com precise/main i386 Packages
Ign http://extras.ubuntu.com precise/main TranslationIndex
Hit http://in.archive.ubuntu.com precise Release
Hit http://archive.canonical.com precise Release.gpg
Hit http://in.archive.ubuntu.com precise/main Sources
Hit http://archive.canonical.com precise Release
Hit http://in.archive.ubuntu.com precise/restricted Sources
Hit http://archive.canonical.com precise/partner Sources
Hit http://in.archive.ubuntu.com precise/universe Sources
Ign http://extras.ubuntu.com precise/main Translation-en_US
Hit http://in.archive.ubuntu.com precise/multiverse Sources
Ign http://extras.ubuntu.com precise/main Translation-en
Hit http://archive.canonical.com precise/partner i386 Packages
Hit http://in.archive.ubuntu.com precise/main i386 Packages
Ign http://archive.canonical.com precise/partner TranslationIndex
Hit http://in.archive.ubuntu.com precise/restricted i386 Packages
Hit http://in.archive.ubuntu.com precise/universe i386 Packages
Hit http://in.archive.ubuntu.com precise/multiverse i386 Packages
Hit http://in.archive.ubuntu.com precise/main TranslationIndex
Hit http://in.archive.ubuntu.com precise/multiverse TranslationIndex
Hit http://in.archive.ubuntu.com precise/restricted TranslationIndex
Hit http://in.archive.ubuntu.com precise/universe TranslationIndex
Hit http://in.archive.ubuntu.com precise/main Translation-en
Hit http://in.archive.ubuntu.com precise/multiverse Translation-en
Hit http://in.archive.ubuntu.com precise/restricted Translation-en
Hit http://in.archive.ubuntu.com precise/universe Translation-en
Ign http://archive.canonical.com precise/partner Translation-en_US
Ign http://archive.canonical.com precise/partner Translation-en
Fetched 72 B in 7s (10 B/s)

HTH