It is unlawful for any person to transact business in this State as a broker-dealer or agent unless he is registered under this chapter or exempt from licensing under this chapter. It is unlawful for any broker-dealer or issuer to employ an agent unless the agent is registered or exempt. The registration of an agent is not effective during any period when he is not associated with a particular broker-dealer registered under this chapter, or a particular issuer. When an agent begins or terminates a connection with a broker-dealer or issuer, or begins or terminates those activities which make him an agent, the agent as well as the broker-dealer or issuer shall promptly notify the securities commissioner.