In the case of a regulated investment company which distributes during the taxable year an exempt-interest dividend (including exempt-interest dividends paid after the close of the taxable year as described in section
855), that portion of any amount otherwise allowable as a deduction which the amount of the income of such company wholly exempt from taxes under this subtitle bears to the total of such exempt income and its gross income (excluding from gross income, for this purpose, capital gain net income, as defined in section
1222
(9)).