Former leader Pinto da Costa enters Sao Tome race

* Post-independence leader to run for president again

* Candidates lining up for island nation’s July 17 poll

SAO TOME, May 25 (Reuters) - Sao Tome and Principe’s former president Manuel Pinto da Costa said on Wednesday he has registered to run in a July 17 presidential election as an independent candidate.

Pinto da Costa, 73, was the former Portuguese colony’s president from independence in 1975 until 1991, overseeing a single party, socialist system in the tiny island nation, which lies in the oil-rich Gulf of Guinea.

President Fradique de Menezes is nearing the end of his second five-year term so he cannot stand.

Pinto da Costa, an economist, told Reuters he had registered on Tuesday as an independent, although he is considered the spiritual leader of the MLSTP-PSD opposition party.

The former president, who was defeated in the 1996 and 2001 elections, has a close relationship with President Jose Eduardo dos Santos of oil-producer Angola, who is likely to back him.

Pinto da Costa goes into the race as a strong rival to Evaristo Cavalho, the speaker of parliament. Three other candidates, including Aurelio Martinez, the leader of the MLSTP-PSD, are set to contest the poll.

Sao Tome is struggling to convince oil majors to invest in its energy sector after initial excitement over the Gulf of Guinea nation faded amid disappointing results from wells.

Nigeria has a joint oil zone with Sao Tome but Angola is increasingly involved, with state-owned oil company Sonangol planning to invest more than $12 million this year to modernise the port and airport. [ID:nLDE73R246]

Reporting by Ricardo Neto; writing by David Lewis; editing by Michael Roddy