Zuma said he had instructed Finance Minister Malusi Gigaba to ensure unity and stability within his new portfolio, from which he fired Pravin Gordhan last week in a step widely blamed for the downgrade.
"We also wish to emphasise that Government has been, and will remain, committed to a measured fiscal consolidation that stabilises the rise in public debt," Zuma said at the launch of the Trans Africa locomotive at Transnet's factory in Koedoespoort.
Zuma thanked Gordhan and Mcebisi Jonas, who was fired as deputy finance minister last week, for "their contribution to strengthening the National Treasury and the Finance portfolio" but said he believed reshuffling the Cabinet has injected fresh vigour into the executive.
He noted Gordhan was formally handing over to Gigaba on Tuesday morning.
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"This shows the maturity of leadership within the governing party... A united and stable National Treasury and Finance portfolio as a whole is absolutely critical for government and the country.
"I have directed Minister Gigaba and Deputy Minister Sifiso Buthelezi to immediately prioritise unity, peace, cohesion and stability within the finance portfolio."
Trade union federation Cosatu on Tuesday called on Zuma to resign, days after the ANC's other alliance partner the South African Communist Party, did so.
AFRICAN NEWS AGENCY