How to replace OpenSSL methods with net ones

Following code in PHP is used to sign data. using private key in pem format. 
    $key = openssl_pkey_get_private ( "pemprivatekey", '' );

openssl_sign ("string to sign", $signature, $key);
 
 
How to convert it into C# .NET ? Which net methods shoudl be used instead of openssl ones ?
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